My secondary school didn’t have a uniform, you could wear what you liked as long as it wasn’t a football top or offensive to anyone* and I can’t say I ever noticed kids getting bullied for not wearing ‘cool’ clothes.
*Although this didn’t stop having to sit behind a guy in an exam once who a massive middle finger on the back of his tshirt with a slogan emblazened above it telling me where to get to in massive letters.
Edit: Actually yeah there was a couple of kids got bullied for not having ‘cool’ clothes but I don’t think it was ever solely about the clothes they were wearing, more likely the bullies just seen them as easy targets.