I went through K-12 before computers arrived. Today, there is no improvement in math, reading, writing, history, science, etc., at all in the output of schools.
IMO there should be no technology in school classrooms until high school. Kids do not need computers to learn K-8 subject matter.
The problem is abuse of the tool, as you describe. In my primary school, we were taught how to use Microsoft Office by using a workbook the computer teacher licensed to the school. We had some of the best computer resources amongst local schools at the time, yet learnt almost nothing about computers. It took me until university to write a line of code.
I understand your sentiment about the opportunity costs for those with computing knowledge, but it wouldn’t be too hard to require teachers to do a basic computing unit at university. In the same way the best mathematicians are not usually school teachers, yet “regular” teachers seem to perform sufficiently.
But in general, why should we expect much improvement? School is mostly still a bunch of kids sitting around an adult, needing to concentrate. Having a computer means you don't have to lug around as many books, but it doesn't free you from having to read the books and do the exercises.
I'm not sure anyone was expecting a large gain from having the computers in the school to begin with, it's one of these things that happens because it gets cheaper as tech gets cheaper.
They sure did. I remember one huge levy for "Technology in the Classroom". All kinds of magical improvements in learning was the sales pitch.
There was no improvement at all, and all the promises were forgotten.