3:15 PM PT -- In court docs, Harvard-Westlake officials say Martin checked into a mental health facility following his arrest ... but he might be out already and the school doesn't want to take any chances.

School officials say the fact Martin was arrested with a gun in his possession coupled with the fact he lives less than 45 minutes from campus is very concerning to faculty, parents and students.

The restraining order request was granted -- and Martin has been ordered to stay 150 yards away from the school.

Ex-NFL player Jonathan Martin's alma mater wants to make sure he comes nowhere near the famous high school after his arrest with a shotgun.

Harvard-Westlake High School filed docs Thursday to get a restraining order against Martin. As we reported, police detained him last week after he posted a photo of a shotgun and ammo, and tagged the names of 2 former Harvard-Westlake classmates -- as well as 2 of his Dolphins teammates, including Richie Incognito.

Out of precaution, the private school campus was shut down when Martin posted the photo. Police eventually located him and detained him. He was carrying a gun with him at the time the police caught up to him.