A madcap romp involving a string of zany shenanigans escalated into full-blown hijinks Saturday at Croydon Preparatory Academy, an exclusive private school in Wallingbrook. 'These nutty kids are driving the board of directors absolutely bonkers with their wild antics,' said school headmaster Charles Croydon III. 'I don't know which is screwier, the loonball goof-ups or the cornball japery.' Paramount Pictures has paid $3.4 million for movie rights to the story of the students' over-the-top hijinks, which the studio plans to turn into an outrageous send-up, expected in theaters in late 1999. 