GWEN IFILL: They have spent $35 billion so far tackling malaria and AIDS and Ebola abroad.

But here at home, their laser focus has been on education reform, which has catapulted them into the middle of a 2016 political debate.

I sat down today in Seattle with Bill and Melinda Gates.

Melinda, Bill Gates, thank you for joining us.

MELINDA GATES, Gates Foundation: Thanks for having us.

GWEN IFILL: For better or worse this year, education is a campaign issue. You can say that is good news, that's bad news. How do you see it, Melinda?

MELINDA GATES: I think the fact that education is part of the public discourse in an election year is incredibly important, because education for us in America is the bridge to equal opportunity.

And so, if it's not working for everybody, we need to be discussing that as a nation. So, in that sense, it's actually a good thing.

GWEN IFILL: But the discussion is all about Common Core and about whether standardized testing, and however you choose to define Common Core, is the right idea.

Do you worry that the whole issue that you have pushed to support, this idea of raising achievement through standardized testing and other ways, do you worry that politics is obscuring Common Core arguments?

BILL GATES, Gates Foundation: Well, it is concerning that the facts about Common Core are often obscured.

The Common Core sets high standards for what math, reading and writing kids should learn in high school, and it helps get the progression down, so that even if a kid is moving from state to state, if they're using online material, it's all benchmarked the same way. And if a kid graduates from high school, they will know that they don't have to go and take remedial classes.

So, it's a very important advance. It's a standard that is allowing for a lot of innovation, where people build elements that connect up to the Common Core. So, we're seeing great results. Kentucky was the first state to go ahead with it. They're actually the state that's seen the most improvement in a lot of their test scores, and even in their high school graduation rates.

So, it's rolling out. It's a foundational piece that will help improve things.

GWEN IFILL: How did the worm turn on this debate, from so many states adopting it so quickly, to so many people, especially Republicans, saying, if you endorse this, it's a disqualifying feature?

MELINDA GATES: Well, I think it's important to look back.

It was the state governors and the state superintendents who, in 2006, said, this is right for each of our states. This is what we want, because if these are standards are set properly, we know that our kids are on a learning trajectory to learn what they need to know to be part of the knowledge economy.

So, that's when it was set. There has been a lot of political debate about — and people have confounded it with, is it federal control vs. state control? No, it's states who are deciding this. A few states have actually rolled back the Common Core, but it's interesting. What they have put in, in place of it is 95 percent the Common Core. It just is a different word.

But 42 states and the District of Columbia are still doing the Common Core, because they know it's right. And more important, quite honestly, than the political debate that goes on is what's happening with teachers in the schools.

When you survey teachers across the nation whose states have Common Core in place, they say, we like it. It's hard to implement, but we know it's the right thing for our students. Our students are learning the things that they need to learn.

So, they believe in it. And so, in some ways, the political discourse just isn't, thank goodness, trickling down to the reality of what's happening in schools.

GWEN IFILL: How have you been able to — or have you been able to measure that these higher standards have led to hiring achievement?

MELINDA GATES: Well, so, Kentucky is a fantastic example. They're the first state that put the Common Core into place statewide.

And what we're seeing is that their graduation rate has gone up, and that's incredibly important. But even more important is the college readiness rate. And that is, before — before they put the Common Core standards in, 34 percent of kids who graduated from their high schools were actually ready to go to college.

Today, that number is 62 percent. That means that kids are going into college and they're succeeding. They know they're ready to go and they're not having to be remediated and then dropping out. That is a profound change in just a few years.

GWEN IFILL: Let's talk about what's happening here in your hometown of Seattle, where the — there was a recent teachers strike, in part over some of these issues. And a judge struck down the charter schools that were supposed to begin here.

I wonder whether that — this example makes you rethink how you can sell what it is you have.

BILL GATES: Well, most states do have charter schools.

And although only about 5 percent of kids go to those schools, they have been laboratories where a lot of good things have been learned. For example, this idea of, how do you give teachers more feedback, how do you help teachers get better, a lot of charter schools really invested in doing that. And then — and we are seeing some of those now be adopted broadly, so that all students can benefit from them.

We are very disappointed that courts saw that the way that the charter referendum that the voters approved, the court said that the way it was funded was inappropriate. And so we're hopeful that charters will be restored in the state. It's possible they won't. That would be a real disappointment to us.

We're among the people who have invested. We have got 1,200 kids in charter schools whose fate are up in the air. Some people are trying to get those defunded, which is — for those nine schools that are in place, you know, that seems like a bad thing to want to shut those down.

But we do have charters in most states, and we think they can be a place where good learnings take place.

GWEN IFILL: What about the students who are opting out of these tests, especially here in Seattle, who are just saying, I don't want to take part in this, there are too many tests?

MELINDA GATES: Well, I think you're hearing that from some students and some parents.

And I think, when new assessments go in, you have to say, what assessments are we taking out? We have to have assessments. They are very important tests to know whether kids who are learning what they need to know against the Common Core standard, whether in fact they are learning the material, so we know by the end of the year.

But I think some of those are also myths about the numbers of kids dropping out. If you look at Louisiana, they talked about thousands of kids dropping ought. It turns out it's less than 1 percent of the kids who dropped out of the state tests. That tells you how important parents and students know they actually are.

GWEN IFILL: When you say that some of the numbers have been skewed, is that because teachers who don't want to be subject to those tests are the ones driving some of this unrest?

MELINDA GATES: Well, I think you get both parents and teachers who get concerned. I would be concerned if my child was tested too many times in the year.

But I think, sometimes, what happened with the Common Core is, it got rolled out. The standards — the teachers agreed with the standards. They believed in them. But then the tests came out very quickly thereafter. And teachers will tell you, teaching to the Common Core is hard. They have to step up their game in the classroom. And it takes a while to get that implementation down. And they need a few years before the testing begins.

So there were a few states that, quite honestly, went too fast with the tests. And I think that's where you started to see some of the pushback.

GWEN IFILL: So, what's the fix?

BILL GATES: Well, in fact, tests where students are told what they got wrong, and they use it as an opportunity to learn and see, OK, how should this concept work, tests are a very good thing done properly.

So, it would be very unfortunate out of this if people thought, oh, we shouldn't test students, we shouldn't test doctors, we shouldn't test drivers, you know, tests are kind of this bad thing. You can make mistakes, particularly how many tests do you have that are those summative tests at the end, how — are those really that beneficial vs. the classroom time?

But, overall, testing is a fundamental piece. And it should be about making them better, instead of saying, OK, let's just take the day off.

GWEN IFILL: In the years that you have been undertaking this enterprise to fix America's education system, what would you say has been the least pleasant and the most pleasant surprise that you have picked up along the way?

BILL GATES: Well, it's always so encouraging when you go in and see a great teacher at work, whether that's in a charter school or public school, to see that energy and the sense of potential those kids feel when that teacher is not only educating them, but giving them confidence.

Then, when we see things that are designed to help teachers improve, that are, you know, pretty good, but there's some fear about how those systems are run, when those are shut down, and teachers aren't getting any feedback at all, that's disappointing.

You know, we need to get to a point where teachers really love the professional feedback they're given. And that's a struggle, to design those systems, to keep them in place, until they get so good that teachers wouldn't want to live without them.

MELINDA GATES: I think the most encouraging thing is, yes, when you see great teaching happen in the classroom, and we see great teachers across the nation.

And I think it's encouraging when you hear from the kids what a difference their teacher makes to them, and they say, this person believed in me. This person helped me with my work. It was hard. It wasn't easy. But he or she believed in me. And they helped me know that I could do this.

When you hear that, and you hear it from kids across the nation, no matter where they're from, you say, wow, that's encouraging. We know it's possible. We know great teaching happens and is possible. Let's figure out how to make sure that all teachers can be fantastic teachers and get the feedback they need, so they keep improving at their craft. And that's exciting.

GWEN IFILL: Melinda Gates, Bill Gates, thank you both very much.

MELINDA GATES: Thanks, Gwen.

HARI SREENIVASAN: Next week, Gwen will have a report from Seattle that features one of the prominent voices that has been pushing back on the Common Core and encouraging parents to opt out of testing.