As far as I'm aware there isn't really a difference, in school we were always told gwisg/wisg ysgol. the literal translation for "dillad ysgol" would be School Clothes, and "gwisg ysgol" would be Schoolwear. but both are acceptable to describe school uniform.

I'm not sure, but I think that "dillad" is for clothes and "gwisg" is for uniform.

Dillad ysgol is accepted now.