Nearly all of our mentors are college students (thanks a lot GA Tech, Ann, Jeremy, and everybody!). In fact, this year we are just beginning to get a mentor or two from the engineering industry to help out a bit in the design process (Georgia Tech and its students are still our mentoring group). Since we are only a second year team (and only had one senior last year who didnt go to a local college), they aren't alumni of our team so to speak, but its proof that college students CAN make good mentors
As long as you get it cleared with the team/current mentors/sponsors etc, there should be no problem
As was mentioned earlier though, if you're interested in designing and building your own robots, there are a ton of college competitions out there. FIRST is about the high school students, and your job would shift from designing and building the robot to guiding thought processes and supervising/advising on the construction. Anyway, it can still be a lot of fun so they say (hey, they're taking us back for another year arent they?). So, Have fun and good luck!