"An accident under your feet". That's what in-line skates are if you use them to get to school everyday. Sooner or later you're going to get into an accident or a near miss. Don't get me wrong, I love in-line skating too and i ride a motorcycle everyday, so i know the probably of an accident isn't any higher than on a bike. But since in-line skating is popular among teens, doesn't need a license, and has a lot less protection, the hazards are higher.
Regardless, in-line skates are awesome means of transport if you got the skills. Put it this way, it's still way cooler than roller skates!