Well, another way to approach this is to copy their Formal outfit to Outerwear, either using MC or by re-creating it as their Outerwear default at least until prom is over.
But I think I see what is happening here. There is an opportunity that some teens in public schools get that involves repairing the school's boiler. Often for me it fires way after the teen needs any of the school performance points that it offers and not all of them really need the money in exchange for the long time it takes to do the repair job.
I don't know why it's up to a student to do this, but if it isn't getting done then your school has no heat! No wonder all of these sims are dressing so warmly. :p