There are basically three options for food at a Prom; a sit down dinner, buffet style, or finger foods. The facility you choose will usually have set menus from which you may choose. They will make minor changes for you if you like, but for the most part you choose from their banquet menus prepared specifically for high school proms. (Be prepared to ask about special diet situations, i.e, vegetarian or other serious food preparation considerations:"Gluten-Free" eating at www.aglutenfreeguide.com
The most expensive option is usually a buffet, and a facility must be really good at food preparation to make a buffet palatable. Your student committee will usually have pretty strong opinions as to what they want, so let them choose, as long as they stay within the budget you set ahead of time. You will find that at most places the difference between the cost of a sit down dinner and finger foods is negligible, so you might as well go with the sit down dinner. An unlimited soda bar should be included in the price as well as some type of dessert.
Don't be afraid to negotiate. You are the advocate for your school, and facilities expect you to ask lots of questions so that you get what you want.