People have a natural circadian rhythm. It also matters
when you sleep, it's not like you can simply shift your sleep by X hours without consequences.
And experts say that school age children and adolescents usually have a circadian rhythm for staying up till a bit later and getting up later. Our culture interprets this as laziness, because agricultural work had to start early so it could be finished before dusk, but neither the earlier hunter-gatherers, nor most of today's jobs fit that requirement.
This phenomenon is also called "social jetlag", when people are forced out of their normal rhythm and only stop being a zombie around 10-11 AM.
There's really no reason to do this, other than the kinda sadistic reasoning that "we also had to get up early and we survived, you kids also have to learn that life is hard sometimes, that will learn you discipline and build your character".
It's no coincidence that many young people like party late at night (or use the computer) when most older folks already want to sleep. There are some evolutionary hypotheses for the reason why young people have an "owl-type" rhythm: perhaps hunting and guarding their home in the dark was their usual nighttime activity.