First of all, please ensure your child is psychologically alright and is not traumatized after such a horrid experience. Explain to him that unfortunately there are terrible people in the World and give him a few security tips in other to try and prevent an occurrence in the future.
Next is to report the case to Law enforcement as quickly as you can, with any luck the thieves may be apprehended and you will get justice. Be very careful though and study the areas carefully, as such molesters tend to lurk around the environment with a lot of ears on ground.
Next is to lodge a formal complaint to the school. I will advice that you do not mince words and let them know the extent of your disappointment in failing to protect your boy. At the very least such a stern complaint with the threat of legal action will likely attract compensation or at the very least ensure the school becomes more alert in terms of security.
First you go to a doctor to protocol the damage on your kid. You might need that later. Then you contact the school and the First thing you expect them to do is to council your child, pay medical bill, and do something about the person who assault your child. And you make it clear to them that lack of action will result in you blowing this poop out of proportion, involving the curatorium, police, and media.
If they see you're holding all the cards and seem crazy enough. They will do something.
Nobody likes to look for New Job, and while school Staff in many countries are nearly unfireable, enough heat might just do it.
Just don't confront the other kid.
That would make you look bad, and for a good reason. You're the adult, Kids don't know better, but you should.
I would go for the big guns, that is to say I would go directly to the police to report the school so that they would be responsible for finding and punishing the culprits. Because after all, the people in charge of the school are responsible for the safety of each child, so it is unacceptable to allow this type of act inside a school.
Another thing is to speak clearly with the child and support and defend him or her to the death, of course without telling him or her to take violent attitudes, but by making him or her understand that he or she has to be strong and take a step forward.
Your child was assaulted by someone and his property stolen from him. This is a very serious case that should not be taken lightly. It's the school responsibility to protect your child for the period you place the child under their care. Since the school is negligent enough to allow such to happen to your child, you should go to the school to first make a report of the incident and demand to see the person that assaulted your child.
let them understand that you're not happy with the way they left the child you placed under their care to be molested and threaten them with legal action if they refuse to provide the person that assaulted your child and return his stolen property. Don't allow it to just slide without doing anything about it because it may happen again if you do nothing about it now
So, the schooling atmosphere is not good.
First keep your child motivated, keep his attitude safe. First he must understand getting beaten up by some bad guys is not the last thing in the life. We are not heroes in cinemas to beat all the bad people, but we have to be face any type of situation in our life. He must feel brave, he mustn't lose his confidence.
Then make complaint to the head of the school. Call their parents and guys who involved in the criminal activity. Just analyse the incident, inquire clearly. You can judge them, if they are normal students and done a mistake, or they done purposely with some criminal attitude.
If situation demands make a complaint to the local police.