I work at a high school and I have seen all sorts of fundraisers. One that typically works well at this time of year is a Valentine's gram. You charge $1 per "gram" and give each person a card to fill out. On Valentine's Day you deliver the card with a carnation or a heart-shaped sucker. Students tend to send out 2 or 3 each. Your start up costs are minimal and you are doing pre-sales so you get the money all up front.
Another great fundraiser that our school did this last weekend was a Battle of the Bands. The students form their own bands and have tryouts that are reviewed by a panel of staff & students. Obviously you are hoping the music is original (and also clean).
The panel chooses the top 10 bands who then get a couple of weeks to practice and then they all perform on one night. They charged $5 - $7 a person at the door and each paying customer got a ticket. Each band has a box with their name on it and at the end of the night every person puts their ticket in a box as a "vote" for their favorite band. This way the crowd gets to be the judge and the winning band got a $100 prize.
Between the door ticket sales and a snack bar, our school's senior class raised over $3000 for their prom. They held it in the school gym, so the location was free and they were able to use the school's sound system. There was very little start up costs...just the time invested in getting the bands picked and ready to go.
Good luck and let me know if you need any other ideas. I work with ASB so I am always involved in the fundraisers.