Mammoth is 5 hours away and it goes without saying is awesome. Depending on what school you are looking at several schools have active ski teams that compete every other weekend, so it is easy to find people to go with.
Mountain high is the closet resort and is 1- 1.5 hours from LA. It is a small mountain with three resorts: one is green runs and sledding, one has longer intermediate runs that few people ride with not park and west resort is the main resort. West resort has jumps and rails on most runs. It is close and has night skiing everyday. Mostly snowboarders and gapers to be found at Mt high. Weekends are like Disneyland but they usually have creative jibs. While I was in college I mostly rode here because it was easily to go to before class.
Bear Mountain is an all mountain park. It is 1.5 to2 hours from LA. Snow summit is next to it in Big Bear and they are owned by the same company. Both mountains are larger than Mt high and Bear has better jumps. I think they have too many ride on features but if I have as full day to ride I would go to Bear over Mt high. They have really awesome snowmaking that makes up for lack of snow most years.
Mt Baldy has the best steeps in southern California and takes less then and hour from LA. It does not have much snowmaking or park but when it snow in February you want to go to baldy.
Finally there are a couple other small resorts like Snow valley that I am not going to talk about because they suck and you do not want to go there.
If you have any questions feel free to send me a message.