Whose gun is it?
Lots of good responses here.
Next up: should he be allowed back at school, and if yes, any conditions or restrictions?
If it’s the father’s gun, it certainly shouldn’t be taken unless as evidence. The father should be advised to secure the gun from the teenager and advised of his culpability in any crime or civil action.
“Next up: should he be allowed back at school, and if yes, any conditions or restrictions?”
On the one hand there’s not enough information. What’s the rest of his conduct been like?
On the other hand, his father should put some restrictions on his life for being, at a minimum, stupid and his good standing with everyone should be conditioned on future behavior.
On the gripping hand, everyone concerned should be watching him to ensure nothing bad happens and that whatever is needed to prevent atrocity or tragedy does happen. That last includes possible incarceration, mental health help, stopping any bullying that may have occurred, and formal or informal counseling to help with whatever problems he may be having. And of course nothing extreme may be needed if it was just a joke.