I think the party might be too large-scale, and it would be difficult to manage that many people, at least at first. Maybe the people who didn't get invited to any parties ended up gravitating toward whoever claimed to have a fun alternative event, and that's where the magic game comes in. Then people who come in late can be handled a bit easier than starting out with a big number of people.
If someone wants to play an older character, like a dorm supervisor or something, that would work, but why a professor or something (unless it's someone's parent) would be there like that is a tougher question to answer. Someone of college age with a high-school age sibling or cousin could be 'forced' to let them tag along and that would be how two people of mismatched ages could end up there. Someone with a crush on someone else might just follow the object of their unrequited affections, or two people actually in a relationship could go together. Whatever anyone might prefer, these are all possibilities. I've already set up the character sheet formats with some age guidelines so that should help.
How does this sound, by the way: The game's directions say to play with the lights down low, or off. The players don't really care for the idea, but once someone throws the dice, the lights mysteriously either dim or shut off. Coincidence? Heh heh heh...Planting the seeds of suspicion in their minds that things aren't all what they seem.