Yeah. It's a damage limitation exercise - sure, some kids are mature enough to run their own lives at 15, but many are not - and you can screw your life far more comprehensively by making bad choices at 15 than at 25 (or 35 or 45).

My impression is that kids that are that mature at 15 find non-school ways to improve their lives/skills/education. The rest need the extra time (and of course for some, no amount of time is enough).