Translation:A school uniform
You are correct, but that's not the point. There needs to be clarification in the translation because in English we would use the article or possessive: "Are you wearing a/your/someone's/etc. school uniform?". I think the problem is figuring out contextually when does "school clothes" become "uniform".
That is right. "Dych chi'n gisgo gwis ysgol?" is "Do you wear a school uniform?" Please understand that "are wearing" would be done differently in Welsh as they have their own form that translates to our present progressive. There is no indefinite article in Welsh, so when one is needed in the English translation we must just put it in. "school clothes" is "dillad ysgol". "school clothes" should be incorrect for "gwisg ysgol", but Duolingo was generous at our first glimpse of the word. We could report it as not correct if you feel it is misleading for later in the full sentence.