I would suggest that you get them to dress as you would to go to a wedding for formal nights and as they would to to to a very good restaurant for informal nights. Definitely no jeans or grunge...Hope that helps Ida Dow.
Son is 12 this year - quite happy to wear dark Blazer and dark trousers with white shirt and bow tie,black shoes for formal nights.
Informal nights, smart trousers and shoes, buttoned up smart shirt seemed fine, might have had a dark buttoned down collar shirt open necked as well.( as you can tell wardrobe not my dept!)
Our 2 ,( also have a daughter aged 16), seem to like dressing up and fitting in at night -quite right too as it all adds to the occassion and contributs to making it a bit of a glamorous evening for all concerned.Also we have some really nice family photos from those evenings.