TMZ spoke with a couple of Crooks' ex-classmates from Bethel Park High School -- who were in his graduating class -- and both of them, who asked not to have their names released, tell us they knew him as a loner, but not one prone to violence.

We're told Crooks was seen as quiet and keeping to himself in class ... as well as during lunch, when we're told he'd often sit and eat by himself.

These classmates tell us he definitely wasn't a popular kid by any means -- but that he was polite and soft-spoken, not to mention incredibly bright, having been in AP classes for advanced students.

The one thing neither of these former BPHS classmates can recall witnessing is the reported bullying that Thomas was apparently subjected to while in school. While some videos have surfaced backing the notion he was picked on, these 2 students tell us they never saw it.

Also of note ... we're told these students didn't see much of Crooks for the better part of 2 years -- once COVID hit in 2020, they were mostly remote ... just getting a chance to return to school late in 2021, at which point they were all graduating.

Either way, both students say they were shocked to hear Crooks attempted to assassinate Donald Trump, because they never saw any warning signs.

Another interesting person who shed light on Crooks was Xavier Harmon -- who was the guy's teacher from 2019 to 2020 at the Steel Center for Career and Technical Education in Jefferson Hills, PA ... another institution Crooks attended as a teen.

Mr. Harmon tells TMZ that he never saw any sign of aggression or hostility from his pupil -- and while he acknowledges he was a bit of a lone wolf/black sheep ... he didn't have any enemies to his knowledge, noting that the kid was quite talented with computer technology.

He also says he never heard Thomas talk politics in class, nor did he hear anything about guns -- even though it's clear he did have an interest in that outside of school.

Harmon says he never saw Thomas get bullied, and had no reason to think he'd ever do something like this. Due to his small, slight frame, Harmon says he jokingly nicknamed him "Muscles."

His classmates and teacher's memories aside ... the fact is, Crooks did try to assassinate Trump this past weekend -- taking shots at him from a rooftop during the rally in Butler, PA -- narrowly missing him, but killing firefighter Corey Comperatore, who was in the crowd.

Of course, some other classmates have painted a different picture of Crooks as an arrogant know-it-all, and a Trump hater.

It's clear he had varying degrees of interaction with other students, and others outside of school ... which explains the disparate descriptions of the then-teenager.