DS attended ski days with his middle school when he was in 6th grade. He was 11 y.o. at the time. There was some teacher supervision, but I'm sure it wasn't very close supervision. He had already taken ski lessons though, so from a skill perspective, I wasn't concerned about using the lift or skiing ability. I was a little concerned about the other kids, who didn't have the same skill level or who weren't as responsible as he was.
If the child is able to ski, then my concerns would be similar to biking or riding public transit on his own. Is he responsible for sticking to the route we've discussed, is he likely to "goof around" and create a risk to himself or others, if he did run into a problem does he have the skills and resources to manage on his own and/or will help be readily available?