Hello again banksy: Thanks for that. This is an odd situation indeed.
From what you say, the school have suspended you (indefinitely), they've made you aware (very vaguely by the sound of it) that a complaint of assault has allegedly been made against you, they've not given you any further details of this allegation, and have advised you to sit at home to await contact from the cyps and the Police within about the next 4 weeks.
To be honest, I'm at a loss to see why you've been suspended. Given that you say you were there as a guest, and I take it that the prom was not even on school premises, then any allegation of an assault would be a matter between whoever made it and the accused person, not the school.
I can only assume that the pupil concerned has reported the allegation on Monday at school? That being the case, the head teacher should have advised the pupil that if he/she wished to pursue a complaint, they should have gone with their parents to the Police Station nearest to where the alleged assault took place to make an official complaint.
You should have been fully informed by the head teacher of what had happened for, unless you were arrested or questioned about the allegation, the school should not be suspending you. I don't know on what authority they've done it.
You really need support from somewhere. You cannot be expected by the school to simply sit at home while everything is done behind your back. That is not on. We're in the 21st century, not the Dark Ages.
I believe you've said that you have a union rep? In which case, get he or she to make some urgent enquiries on your behalf to find out what's going on. If only for your peace of mind.
I won't go any further at the moment. You've obviously got enough on your plate. It may help if you can update us as to what's going on, as I believe that some of us could probably give you further advice. The