Because there is this one girl in my english class and she has a nice ass in general, like im talking its got good volume and almost perfect shape, and when she wears these specific black tights, it makes her ass look so good

It sounds like you're really appreciating her style and how she carries herself! It's natural to notice attractive qualities in others. Just remember to treat everyone with respect and engage in conversations that focus on their personality and interests as well, rather than just their appearance. If you feel comfortable, you could get to know her better and see if you have common interests.