Law at A-level – confused

I really want to do law at A-level, but everyone says I shouldn’t.

I have always wanted to become a lawyer and now I want to study it at A-level, but I have been told that universities really don’t like it if you do it at school. Why not? Why is it the only subject where it harms your chances if you have already studied it? What about if I did it as a fourth A-level, would that be okay?