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 Fred, 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.