Thomas Matthew Crooks Nicknamed 'School Shooter' Years Before Trump Attack

Thomas Matthew Crooks was branded a "shooter" by his peers years before his assassination attempt on Donald Trump ... so says a former classmate.

Julianna Grooms -- who attended Bethel Park High School with Trump's would-be assassin -- has shed more light on the bullying Thomas faced before his deadly attack at Trump's rally in Butler, Pennsylvania.

As Julianna told The New York Times ... Thomas was labeled "the school shooter" by his fellow students -- who apparently often teased him for his poor hygiene and body odor.

Per Julianna, Thomas was an "easy target" for bullies thanks to his wardrobe of wide-legged pants and "SpongeBob SquarePants" shirts. Thomas, who reportedly tried and failed to join his school's rifle club due to his poor aim, eventually ditched his signature style ... opting instead to wear camouflage.

Julianna says Thomas often kept to himself in high school ... avoiding any and all attention. Her anecdote further corroborates reports Thomas was bullied as a teen ... with TMZ previously sharing footage of TMC being hassled in class by another student.

In another video, Thomas is seen making lewd jokes about his penis size as a classmate makes bunny ears behind his head ... one of the first times we've heard his voice on camera.

Like we first told you .... Thomas was shot in the head and killed after opening fire on Trump's rally on Saturday, where he killed one attendee and left the former Prez injured.