This is my take on the topic: PCs became an off the shelf product and the case for special school computers diminished.
As Thomas says, in the first half of the 80s several countries had their own big budget projects to introduce computers into the education. Custom hardware was built. In Sweden where I live we had Swedish made computers like Telenova Compis https://en.wikipedia.org/wiki/Compis, which was developed specially for education and ABC 80 https://en.wikipedia.org/wiki/ABC_80, which was sold for general purpose but commonly used in education. The problem was that the computer business moved fast (as always) and these computers were obsolete in a couple of years. Many of these machines either used proprietary operating systems or operating systems that are now obsolete e.g: https://en.wikipedia.org/wiki/CP/M
When the projects to build ownspecialized computers and use them in schools where finished and the PC had arrived as the standard I think that there were no new money assigned for using specialized hardware. Education generally do not have huge budget and specialized hardware is resource intensive to develop. It is always cheaper to buy a standard configuration than a. The fact that the old systems were more or less proprietary made their software unusable and their legacy faded. PC clones were just too cheap and the added value of specialized systems did not meet the extra investments needed.
Apple has been successful in selling computers to schools as they have special discounts for the education sector. In my children's school they use IPads and Mac laptops so. I would say that we are moving away from the PC form factor in education.