Omar Mateen Threatened to Shoot Up His School in Fifth Grade!!

Omar Mateen terrorized his Florida grade school by threatening to go on a shooting spree ... according to multiple former classmates.

5th grade classmate Leslie Hall tells us Mateen told a group of kids at Mariposa Elementary he was going to bring a gun to school and kill everyone. She says it was not received as a joke and the kids immediately told a teacher, and Mateen was suspended for 2 weeks.

Hall doesn't recall exactly what prompted the threat, but does remember her mom having multiple meetings with the school faculty about Mateen's constant bullying of her and other students.

The behavior issues continued into middle school ... according to Hall and other classmates who say he threw chairs and spat at teachers.

As it's been reported ... Mateen also shocked high school classmates on 9/11 by bragging that Osama bin Laden was his uncle.

Mateen's former classmates all agreed the warning signs were abundant at an early age ... and they all wished more had been done in terms of discipline or counseling.

We reached out to the school district ... they didn't want to comment.