I agree that it can be difficult to balance mentorship with academics, and I do agree that academics should come first. It's a good idea, wherever you go, to take the first semester (or first year) to figure out your schedule and what you can handle before you get too involved in *anything* -- including clubs, sports, and FIRST.
However, that said, mentoring as a college student can be really rewarding. In engineering in particular, it gives you a lot of perspective. Engineering can sometimes be overly theory-based and it's easy to lose sight of what it truly is -- design, teamwork, fabrication, testing, all the fun stuff that FIRST taught us about. And it's incredibly valuable to be on the "teaching" side of the equation where you must truly understand something in order to explain it to someone else.
I go to an extremely challenging college where working hours are pretty much any time Outlook can send a meeting request for both students and professors and where committees, clubs, and classes easily eat up your time. But I (and other students) have made time for things like FIRST. It does require the sacrifice of some things (like social life, or like getting an A instead of a B+ on something), but it is doable without failing out --
because I have figured out my limits
Our group has found it particularly helpful to be working with a team with a very involved high school teacher (who can take care of the logistics of workspace, funding from the school, registration, and keeping the kids coming) and which, because it's an inner city school, *only* has open hours from 2-4 in the afternoon and, when we can argue it, some weekends. That means no all-nighters, no practically living in our lab, and that we all have to take a step back and remember that we have other things to do. Does it mean we can be one of the hardcore teams? No. But it doesn't mean that both the students and the mentors aren't learning, building important relationships, finding inspiration in engineering and science, and still fulfilling the FIRST vision.
You certainly can't have everything. You can, however, still have a valuable FIRST experience, and provide one for high school students in need of a little push toward the awesomeness of engineering.
<plug for Boston FIRST Regional> And if all else fails, volunteer at competition.

That's a really important job, too, and it makes planning committees happy! </plug>