Do you know what happens when someone makes a threat against a school or anything else such as the President and so forth?
They are investigated, their families are interviewed, the subject is interviewed, etc. to see if the person actually has the motive, means, and ability to conduct said attack.
They are not instantly thrown in jail and they are not arrested. The only way to be arrested for that is for there to be probable cause that the individual actually had the means, motive, and ability to carry out the action.
I quote this because unless there is an immediate threat to the school and Thompson has the ability to do what he may or may not have said, I don’t think he has anything to worry about as far as being expelled. Especially if he has no other documented history of this behavior.
Edit: Sorry BGS that was supposed to be a quote from me. It is not directed at you.