How do you slow/ballroom dance?

Well if this is a casual school dance, you won't have to ballroom dance.
Just maybe "slow" dance.

For slow-dancing, just put your hands on the girl's waist, and she will put her hands around your neck, or on your shoulders.

Just rock right and left with her with the music.

How close you hold her depends on how long you know her, are you friends or gf/bf, or just met.

But usually stay at a pretty respectable distance (if you dont really know her) and let her get close to you if she wants to.

if you actually need to "ballroom dance" like formal "quick step" or "jive" than you got to take lessons, though somehow I doubt you will have to do that at a school dance.

For a school dance, your hold should be more compact than the wide hold of ballroom, which simply takes up too much space to be used on a social dance floor. Nonetheless, the basics are close to the same. You want to be slightly offset to the left of your partner, so do NOT "just put both hands around her waist," or you'll risk stepping on feet. Your right foot should roughly line up with a spot between her feet. Best is if you take her right hand in your left, and put your right hand on her left shoulder blade or, for an even more compact hold, in the small of her back.
You needn't do much with your feet. Many of your classmates will likely be using that "both hands around the waist" hold, and they'll just sway left and right to the music without moving at all, since any step would likely injure their partner. That's not a bad plan, but if you want to add real steps: If your weight is on your right foot, then you can only move your left, and it can only go forwards, backwards, or sideways, so it isn't as if there are a lot of options. Keep your steps small, and rather than clomping around, just slide your foot across the surface of the floor perhaps 8 or 10 inches (or even less). She should be able to feel your motion and the shift of weight from one leg to the other, in which case she'll naturally go with you. Keep it simple and have fun.

http://www.youtube.com/watch?v=SI3WSiTBb5Y