It is bad if he left you a bruise on your arm. If he is from your school, you should report this to the management or the principal for immediate action. If he is from the neighborhood, you can report the hit to the police.
Yes it's bad, if it was intentional, if it was meant to be like a friendly "hit on the shoulder" but accidentally left a bruise (which, if it left a bruise doesn't sound friendly) then no, it's not bad. If it was intentional, report him to principal or police :)