She's 11, so you are buying her clothes. If you don't want her to wear crop tops or anything sheer or short, don't buy them. If she already has them, you can take them away if they are too revealing. You're the mom.
Otherwise, you can't add something (a bra) as easily as you can take away (crop tops). So, I wouldn't fight this. When/if she's uncomfortable, she will wear a bra. If she doesn't like the "proper" bras you purchased, what about a sports bra? You say she's into sports, so many of her friends will be wearing these. They aren't wearing crop tops on the field, right? Other kids just haven't developed much at this age so they aren't wearing anything yet either. If she needs more coverage but doesn't feel the need for support, what about a camisole? It doesn't have to be frilly - just stretchy and with a little coverage from an extra layer under her shirt. Either way, the straps shouldn't interfere with her athletic moves - girls find falling straps really annoying, and I can't blame them.
Is she changing clothes for gym class? Does she have a private changing room for that, or would she be more modest in either of the above items (sports or camisole)? What are the locker room requirements re uniforms and privacy? These are some questions you might ask to get an idea of what the expectations and practices are at the school. But again, sometimes you need to start to let go a little at this age, and let consequences occur. Unless she's exhibiting herself excessively, start letting her take care of her own body.