If you're planning on playing any of the local 24-hour games, a backpack or daypack is almost a necessity. Walking all the way back you your car for shit sucks ass and lets down your teammates who are depending on you to be ON THE FIELD, and bringing day-glo coolers and totes full of crap onto the gaming area is lame (unless you have a tent or mess area). Carry your shit on you and you never have to leave the gaming area, which can be huge depending on where we play.
You don't need a huge pack, either. Something in the 25-35 litre range will do nicely. Think about the stuff you'll need to pack in a day-long (12 hour) game:
raingear
water
eats
bug spray
propane
ammo
ammo loader
extra batteries
sunblock
possibly a wooly-pully (spring/fall) for early morning or early evening
Ranger blanket if you're planning on resting, or to make a quick shelter
You gonna pull a half hour walk back to your car to get your raingear, or to get another battery? What a waste of time, not to mention letting down your teammates.
Besides, get a good peice of gear and you can use it for your school bag.
To give you an idea, I use one of these, which I'll be happy to show you if you turn up to n00b day onthe 18th:
http://www.specopsbrand.com/assets/p..._the_pack.html