It depends on how old you are. You could choose black and white tank tops with a built-in bra (at Justice). But if you are older, then you should use...
Don't worry about them. If it's really bad and making you very uncomfortable at school, you could change schools, but I personally think that the best way is to ignore...
You can wear white pants any season you like. White pants are often made it light fabrics so might not keep you warm in cold weather, but don't let that...