Ah, I was thinking you meant a sports coach or similar, ie a person. My misunderstanding. So it's a group of pupils organising this, booked the coach, booked a venue? Presumably helped by parents.
I think you could talk to the school about how this will be handled, ie when a coach arrives on their property to take the majority away to an after party, leaving a minority of students behind.
Exclusion of a minority like this must go against school policies, eg exclusion can be a form of bullying.
But if it's been organised without any involvement from the school it's possibly a grey area, but it's not unreasonable for them to address the situation with you. I would hope they would want to avoid the end of school being tainted like this for a small group of pupils.
Alternatively, could his friends adopt a screw them approach and book something they'd enjoy, entirely separately. Dinner and an escape room or whatever they're into? Definitely don't want these kids all sat in their bedrooms feeing left out on prom night.