what is another way of saying this?

Well, it needs fixing, because it's probably not the school that is watching TV, and it's probably not the school whose performance decreases. And it assumes that the heavy TV watching is the cause of poorer performance at school. Unless there's research to back it up it might just be that the sort of person who does poorly at school is the sort of person who watches a lot of TV, with no causal relationship between the TV watching and the performance. So perhaps "Students who watch more TV tend to do less well at school."