This is an efficient summary of the social attitude I'm criticizing. People believe that children and teenagers are useless. They believe this because the structure of modern education requires young people to be useless. Every adult in their lives repeats variants of "you cannot and should not be trusted to do real work that other people need", and so young people grow up believing that.
If instead of resenting their presence and demanding that they be warehoused away in the name of efficiency, adults in contemporary society ought to look forward to training young people to be useful. If you take this attitude, and regard every young person as a future ally in your endeavors to maintain civilization to a high standard, it becomes clear that what's being done in most schools is inefficient, poorly-managed, and counterproductive.