Well, people are funding schools through taxes. Then they send their kid to school. Then the school decides their kid is too stupid* to do much, so they get relegated to dumb class and their future career prospects get nullified.
This is how it works in many countries where schools are segregated by learning ability.
I make no judgement on how good or bad it is, but I get why people would be upset by this system.
*or has some mental issues like ADHD or whatever, which a lot of countries do not even recognize as a thing