NFL's Desmond Clark -- Video of High School Tirade ... Over Alleged Racism (VIDEO)

TMZ Sports has obtained explosive video of ex-NFL tight end Desmond Clark GOING BALLISTIC at officials at a Chicago area high school after his son was allegedly mistreated because he's black.

According to police, it all went down on August 29th at Vernon Hills High ... where Desmond's 17-year-old son was blocked from participating in a school lip sync event because of a previous code of conduct violation.

Desmond's son claims an administrator demeaned him in front of other students -- calling him a "criminal" -- so he called his parents who came rushing to the school.

You can see in the video ... Desmond's wife Maria got so HEATED, Desmond had to literally pick her up and remove her from the person she was screaming at.

Desmond -- who played for the Bears from 2003 to 2010 -- comes back into the room and begins to yell. Cops say he was calling out school officials ... claiming his son had been the victim of racism for years and his family was sick of it.

Several people in the room gave Desmond an ovation at the end of his rant.

Eventually, cops arrived to the scene and spoke with the Clarks for more than an hour. They left that day without incident ... but Desmond and his wife were hit with criminal charges in the following days.

Both Desmond and his wife were charged with disorderly conduct -- Maria was also hit with an assault charge. They were arrested on September 11th and were later released.

They have since pled not guilty and are due back in court later this month.

Desmond has since been banned from school grounds -- his lawyer says, "They're afraid of him because he's a big black man."