Because "Do you wear new school clothes?" simply isn't something you would hear in normal conversation. With that in mind, the course expects you to rule that out as a possible answer and instead use "Are you wearing new school clothes?".
Just like these sentences "Dych chi'n bwyta cig?" can be understood two different ways based on context. "Do you eat meat?" and "Are you eating meat?" and it's up to you to figure out which is the right one by using context clues or something of the sort.
That's just what I kinda figure so far from what I've read about it.
You are wearing new school clothes and are you wearing new school clothes translate exactly the same into Welsh. The only difference is the infliction at the end of the sentence which makes it a question. Your voice goes higher at the end of sentence.