Grades do come first. Think about it ... FIRST doesn't graduate you from HS ... your performance in class (your grades) do.
Its all about the individuals striking a balance between their school work (finals, hw, cw, etc) and robotics. For example, in my team we have finals coming up next week so we don't have to worry about studying over the holiday brake, however we do start build season the same time that we start a new semester (brand new classes). Its hard to balance the two. And some people can manage it ... others can not. Personally, the way I see it is: if you can manage being in robotics as much as you can and still get A's or B's (or whatever is your norm) -- that's GREAT!! However, if you are there all the time during build season but your grades are D's and F's ... you don't need to be in the team at all.
Make sure that you stress to your team the importance of keeping up with their grades and concentrate on them first.
__________________
2004 Peachtree Xerox Creativity award winners.
2004 Peachtree Winners, thanks to
1002 and
1057
Team 1369: MINOTAURS
I have to agree ... Dorienne Plait and I are quite a DUO in CD!!! 
"The true nature of the modern condition - how weak we are, how helpless against ourselves in this proud era of so-called progress." - Max Beckmann.
Always Here to Help