A tiny room with a hole in the wall access selling sweets and crisps. I know there was more, but all I can remember is what I was usually able to afford: Black Jacks and Fruit Salad chews, 2 for 1p.
It was run, regimentally, by Mr Eustace, who looked like a quartermaster.
I think that the tuck shop queue was one of the more perilous places in my school: standing in it meant you had some money!