aren't they normally referred to as "formals" or "balls" here?
two countries divided by a common language
(likeajackhammer, Fri 26 Jun 2009, 14:51, archived)

Box socials.

You're right, it's the Americanisation (sic) of our language, 6th forms and colleges all round the country have proms now. It's a bit silly really.
( Devil Duck: ( ' )< and his cunning stunts, Fri 26 Jun 2009, 14:55, archived)