''We have a Nobel Prize winner; a new state-of-the-art library, and $ million (from) Walter Annenberg — and we're best known for the guy who says, 'Whoa!''' responds University of Southern California Steve Yawitz to the news that Blossom hunk Joey Lawrence is a member of the Class of '98. Lawrence, 18, insists he has typical student worries — and then some. ''I get followed home a lot,'' says the stud-turned-studier, who brings two undercover cops to school with him. ''It's sort of scary.'' Former television stars Jeremy Miller (Growing Pains) and Brice Beckham (Mr. Belevedere) are among Lawrence's fellow Trojans, while Jaleel White of Family Matters is at UCLA. But most students are blasé about teen idols. Says UCLA junior GG Heintz, ''It's Hollywood and we're kind of used to it.''