Chicago needs at least 126 new public school principals by fall--so many that the district has hired a professional headhunter to reel in candidates from across the nation. Extra cash is being dangled at candidates willing to take on the toughest public school assignments. The retirement of dozens of baby boomers combined with the expiration of a pension sweetener has created more principal openings in the Chicago school system than at any time in at least a decade, officials say.