In 2003, Jack Black was still something of a novelty act that had not yet worn thin, a careening amalgam of Belushi and Shatner that crackled with boundless energy. School of Rock was perhaps his perfect vehicle (outside voicing animated characters), contrasting his immature absurdity with the repressed sweater-and-checkered-skirt set of a snooty private school and the prematurely adult students there. Black's Dewey Finn was a wastrel, a vagabond, a would-be rocker with modest musical chops and more passion than industry. He surely loved the music more than it loved him, and his bandmate/friend/reluctant host Ned Schneebly (one of the great names in movie history) was giving him the boot from his apartment floor, egged on by Sarah Silverman's spot-on wet blanket girlfriend.
And so Dewey does what all low-grade grifters do (and, let's face it, what John Belushi would have done) and engages in fraud. He impersonates Ned as a long-term substitute teacher at Horace Green, figuring to score some easy bread while sleeping off hangovers and nominally chaperoning a classroom with all-day recess. The scholastically ambitious students in his class beg him to teach them something, so Dewey obliges by lecturing on the only thing he knows - rock music. He proves an impassioned and enthusiastic instructor on this subject he loves, and clearly knows well.
Ever on the make, Finn conscripts the entire class into his new band, and the results are hilarious, mainly due to some wonderful acting by Black's young co-stars. Miranda Cosgrove's acerbic Hermione-esque Summer, Robert Tsai's "Mr. Cool" on keyboards, and the rest of the class are almost uniformly perfect. Of course the entire premise is utterly implausible - presumably for weeks, the upper leadership of the school and the high-powered parents of these children are unable to penetrate Dewey's thin web of deceit. But hey, it's Hollywood.
Dewey Finn is scarcely an admirable soul. He's a liar, a fraud, and if truth be told, endangers his young charges in several ways. But it's also clear that he adores them (probably because he remains a child himself), and he has a gift for connecting with and inspiring them. The embedded lesson here is that the best teachers aren't those with some formal certification, but those who impart both knowledge and passion to their students, and who engage young minds in both the outcome and the process of learning. Of course, it does help if they actually are who they say they are.