Jack Black was in full rock god mode Tuesday night in Chicago during a Tenacious D set -- which served as the perfect setting for a "School of Rock" reunion with one his "students."
Professional drummer Kevin Clark -- who played Freddy, aka Spazzy McGee in 'SoR' -- tells TMZ ... he got in touch with Jack when he heard TD was playing a gig in town, and that Jack made sure he was given tickets and backstage access at the Riviera Theatre.
Perhaps, unsurprisingly, the reunion made for some epic nostalgia ... almost like it was 2003 all over again. We're told Jack and Kevin hadn't seen each other in a while.
Fun fact: Kevin hasn't acted since starring in the film, and says he only got the role at the time 'cause he could actually play the drums. Freddy plays in his own band now -- Dreadwolf -- and gave Jack one their custom shirts, which he threw on immediately.
BTW ... the actress who played Katie -- the bassist in the film, Rebecca Brown, attended Jack's TD show Wednesday night.
You know what that means ... stick-it-to-the-man-eosis is spreading!!!!