A "hard one" is given in middle school to someone when they say or do something really stupid. In order to give a hard one, you must announce it by saying "that's a hard one!" You then get behind the victim and push their head down. The victim can counter a hard one by saying "self serve", and they are allowed to give themself the hard one, as hard or soft as they wish. You can also say "no self serve" before they have a chance to say self serve, to make sure you can get them hard. You can also say "wiitness", allowing others who see this going on to say "witness", to also give the victim a hard one. The victim can call "no witness", given you have not already called it.
In math class, the teacher explained our assignment and then right after, Brett asked her what the assignment was. We then gave him a hard one.