No, a 12 year old will not be put into jail,although he could be put into juvenile detention. There are several resolutions you have asked about which you need to decide between. You can report the conduct to the school, and live with their disciplinary actions; or, you (for the child) can file a complaint with the police and the child becomes a witness for the State in Juvenile Court. Once you make the necesary record with whichever alternative you select, the resolution is out of your hands. As an alternative, you may want to talk to a civil lawyer licensed to practice law in MN to see whether there are any civil actions you may want to pursue. Since a civil action is literally between the two parties, you (assuming you are the guardian) get to decide what the resolution of your civil action will be. Good luck.
This information is offered for educational purposes only. It is not intended as legal advice and you should not rely upon it to decide how to resolve this issue. No Attorney-Client relationship is intended or established by this response. You are faced with a situation where you need to consult with an experienced defense lawyer admitted to practice law in your State before you make any decisions as to how to resolve this issue.
If a juvenile commits an assault, the police can investigate and consider referring the case to the County Attrney's office for a possible Delinquency Petition. I would think that the target of the assault could seek help from the school, the police, or both.
In Minnesota you also have options for Restorative Justice resolutions where there may be a victim, offender, community circle to discuss a resolution that helps make sure that something like this doesn't happen again.