The décor choices you make have only one major limiting factor: finances. If your budget allows you to create a virtual movie studio, then by all means go for it. If your budget is more limited, then you must figure out what part of your budget you can spend on decorations and make sure you do not exceed this amount.
Food is usually tied to the venue, in that almost all hotels and banquet halls have either an in-house catering department or a preferred list of caterers. As with décor, your limitations are primarily financial, with special consideration being given to nutritional concerns. Use your free school resources-your school district will usually have someone on staff who is knowledgeable about dietary issues and can advise you on subjects such as vegetarian meals or special dietary restrictions that some students may have.
Your main decision about food will be whether to have a sit-down dinner, a buffet-style dinner, or just serve appetizers and desserts. Local custom usually dictates your choice in this area. In areas where most people favor restaurant dining before the Prom, it doesn't make sense to spend the money to serve dinner.
The subject of entertainment is greatly misunderstood when it comes to budgetary planning. The mistake some groups make is to blow their budget on the venue, décor, and food, and not have enough left for quality entertainment. This is the biggest mistake that a Prom committee can make, as the choice of entertainment will determine the quality of the evening. If your DJ or band is great, people will say they had a great time. If the entertainment is bad, your teens will remember the event as a dud. I've seen plenty of Proms where the food was not up to scratch but the attendees still considered it a success because of the quality of the entertainment. I've never seen an event with good food and bad entertainment that was considered a great time. Clearly, the entertainment choice is your second most important choice, right behind that of the venue.
In the last two decades, schools have begun to run after-Prom parties, where all Prom-goers are locked into the school building or other community building for the evening to enjoy a variety of activities such as dancing, casino-type games, movie rooms, fortune tellers, caricature artists, breakfast, and raffle-type events. (At the 2004 After-Prom at Kennett (PA) HS, they even raffled off a brand new car!)
Indeed, in schools that sponsor such events, they have almost eliminated Prom night drinking fatalities. Just remember that almost as much planning goes into this event as the Prom itself.
Some sort of a security team is always a good idea at your Prom. My best recommendation is for your group to hire off-duty police officers and have them show up in uniform. A good show of force will eliminate a lot of the problems before they even start, and for those problems that still occur, the officers are there to solve them.