I think that mandatory education serves a very important purpose up to a point. Having said that, I have made the following argument for years:

The typical response to poor education outcomes has been to make school easier. The correct (and liberal) attitude should be to make school harder. They should make high school hard enough that (1) having a high school diploma means something and (2) not having a high school deploma is not an economic death sentense. Then, undergraduate education should be hard enough that a college degree is a mark of high distinction and a graduate degree a rare and impressive thing.

This has a number of benefits. The first is that we don't waste resources keeping people in classrooms longer than is necessary. The second is that instead of being in school, people can be making a productive contribution to society. The third is that, by reducing the time and money needed to get an education, the field will be more level for the poor, those that can least afford to trade 4 years for a piece of paper and 50k in debts.

Instead, we decide that everyone should graduate from high school, making a high school diploma meaningless; and that everyone should go to college, replacing the middle class ethos of self-investment with the old money ideal of personal experience. It is a horrible system that has been set up; an illiberal failure in the guise of liberalism.