'X-Factor' -- High School Choir Claims SABOTAGE

"X-Factor" producers concocted an elaborate scheme in order to GUARANTEE the failure of one H.S. Show Choir ... all for the sake of shaming them on national television, so say the alleged victims.

Members of Hollywood High School choir -- H20 -- filmed a segment for the upcoming season of "X-Factor"

But things didn't go as planned. The leader of H20 tells TMZ ... producers had a sick plan to humiliate and embarrass their crew when they appeared on stage.

They rehearsed an awesome song "Some Nights" by FUN, but at the last minute producers told them the song wasn't on the approved list. Ditto the other song they rehearsed -- "Rolling in the Deep" by Adele.

They ended up singing "Shake It Out" by Florence and the Machine ... and it was a disaster. For starters, Simon cut the choir off before they could get through the song ... because they were wearing normal street clothes instead of their uniforms. But here's the thing -- they say producers insisted that the group NOT wear the uniforms.

But wait -- there's more. The group claims they had to wait 14 hours before taking the stage, they weren't able to rehearse, and they only gave the 14 choir members 6 mics.

The group tells us ... they're demanding an apology because they feel they were purposefully humiliated.

A rep for "X-Factor" tells TMZ ... "The group had the chance of performing 2 songs for judges. The judges didn't think they were good enough to move forward in this competition."

Then adds, "Certain songs cannot be cleared for use in the show and contestants are informed of this in advance and are asked to choose alternative songs if this is the case. None of the judges even commented on what the group was wearing."

Now here's what the group sounds like .... singing "Some NIghts" at TedX in Hollywood.