Children hit by parents get worse grades

A study found that children in England who were hit by their parents have a tendency to get worse grades in school. This could indicate that hitting children tends to lead them to do worse in school tests. Or it could indicate that children who for certain other reasons tend to do worse in school will tend also to be hit by their parents. Is it the hitting itself that does them harm, or the situation that leads to the hitting, or both, or something else?