Just go in and ask the boss or CFI as Shortstripper mentions, don't take a no from the person behind the desk, unless it is the boss! Just be friendly, presentable and most of all keen. You will get all the crap jobs, but it is good experience, and you'll often get alot of people taking you for a blast for free. You will also learn alot by just being there, let alone making contacts who could be useful later on.
At my school we have plenty of people who help us out behind the desk, refuelling etc. and we give them 1/2hr flying per day worked. Seems to work as we have to turn down alot of people!