Did you overreact? Maybe. I think he's made that request without really thinking through how that would look to you, and he's figured there's no harm in asking you to use your sister's uniform. In the round it's not a bad thing that he's willing to ask for things that he wants in this context. Anal and school uniform are pretty unoriginal in that regard and perhaps reflective of your boyfriend's lack of experience (I don't say that in a disparaging way; on any reading at the age of 21 he is inexperienced compared to someone in their 30s or 40s), but it is important that he's willing to keep those lines of communication open. It's important for both of you to be able to do that.
At the same time, your reaction to that is entirely understandable. I've never been asked to wear a school uniform for sex (probably because I'm a guy) and it's not a fantasy that I find to be particularly savoury anyway, but even my skin crawls a bit at the thought of you wearing your actual sister's actual school uniform to have sex in. I think he should have thought about that a bit more before asking. And in reality, whether you shout at him or not, the request has killed the mood anyway, so nothing is happening at that stage irrespective of how you react. So it's all ultimately heading in the same direction.
I don't think you should hold it against your boyfriend going forwards though, and it's probably best to talk about it. Conceding that you overreacted is not a bad thing (even when you may have thought that you didn't), and I'd let him know that it did kill the mood for you. Again, stress that you're not judging him for that because there will definitely be more times in your sex life when on or the other of you does or says something that kills the mood, but do it by way of an explanation for what happened here. Doing that will just allow you to smooth things over and get back to having a good sex life, hopefully with some slightly more original requests in future.