Schools and teachers have ways to make sure improvements don't happen. These include - same pay regardless of performance, not hiring or firing based on performance, and teachers don't want to learn or do anything differently, especially if it's even slightly harder than what they already do. They're not really professionals that improve or have any career growth besides management and accumulating number of years experience. Schools don't care either, nor do governments bother to incentivize them to care.

Except in, say, China, where none of that applies and education works great.