Apryl - I have the same sort of problem. My commute from school (I teach 5th grade) is 40 minutes, so at the end of the day if I cave into the munchies it could mean a disastrous trip through the McDrive-Thru!
I agree - you should keep a good supply of sturdy fruits or veggies. Apples do it for me. Also, pretzels are good, but make sure you bag up individual servings instead of packing the whole bag. If you're hungry enough (or bored enough or stressed enough) you'll end up eating the whole big bag!
I am so proud of you for going about this the right way! Planning is so important...it took me about 30 years to figure that out, and you're only what - 14? - and you've figured it out already!