”You’re not supposed to go back as a cool rock star and sit in the principal’s office,” says John Mayer. But that’s just where he found himself on May 15, before heading out to electrify 1,200 Pennsbury High School prom revelers with a surprise three-song acoustic set. Though not an alum, the platinum seller admits to feeling ”giddy” about rocking the Pennsylvania students — even if the notion wasn’t exactly his. As documented in Michael Bamberger’s new book, ”Wonderland,” which takes its name from the Mayer hit, overachieving class president Bob Costa spent the 2002 — 03 school year trying in vain to book the singer for the tux-and-corsage fest — and the year-in-the-life account of a prom-happy town culminates with last May’s Mayer-less dance. ”I thought if there’s any artist who would play a prom, it would be John Mayer,” says Costa, who got this year’s good news just 24 hours before the big event. Despite being denied a fairy-tale finale for his book, Bamberger — who was ”front and center” for Mayer’s prom show — hopes to rewrite a little history, now that Paramount has optioned the tome for the big screen: ”It would make a wonderful final scene to have John Mayer play the prom in the movie.” What a novel idea!