That's for the 1986 student population with 1986 dollars. If you correct it for the estimated 2018 student population and 2010 dollars, you're already around $190 billion. And that's not counting the cost of building new schools, or factoring in the rise in per-student spending. You're easily looking at well over a trillion dollars when all is said and done.

Also, decreasing the class size to 15 students wouldn't get you very much improvement over current class sizes. It's only for every incremental decrease below 15 that you start to see meaningful improvement.