Rajon Rondo SCHOOLS High School Kids ... in Math

Boston Celtics superstar Rajon Rondo crashed the boards at a Boston high school today ... guest teaching a freshman algebra class ... on his day off.

Rondo -- rolled into Ms. Oshodi's class at Burke high school this morning -- where he taught the kids how to solve for X (it's 14 ... we think).

Rajon didn't stop there ... the 26-year-old -- who attended the University of Kentucky -- also visited nearby Harbor Middle School to preach the value of a quality education.

And get this -- we're told Rondo planned the visits by himself ... without the help of any public relations personnel ... just because he wanted to.

Rajon will be back on the court tomorrow to face off with the Washington Wizards -- a team that's currently 0-2 ... and with the power of math, we are able to determine they're horrible.

Thanks math!