Hooking up can mean anything across the spectrum of love-making, from making love to simply canoodling.
With a senior, definitely not weird. With a junior, not much. Sophomore or freshman is starting to cross the line a bit and pretty much requires you were in a past relationship.
Jumping education brackets is not normally smiled upon. Even a 9th grader and an 8th grader (or wherever your high school-middle school divide is) in their kiddie relationship is a good deal stranger than one up or one down, or even Freshman-Senior. However, that's mostly because of the culture of relationships at that age. The teens all see each other and their peers all day, so there's a sort of social atmosphere they all fit into. However, if the junior is particularly mature or the college student is particularly immature, or if they are tight in some way that goes beyond their respective schooling, nah, there's no issue with that as long as the college student doesn't mind a little ribbing from their friends on-campus.