A CAUSTIC CLASH ON CITY'S SCHOOLS
Democratic mayoral candidate Ruth W Messinger and Schools Chancellor Rudy Crew clash over condition of city's public schools in confrontation that seems likely to help set tone for last 12 days of Democratic primary and perhaps general election itself; Messinger shrugs off Crew's assertion that she is 'denigrating the public school system for political gain'; she visits Public School 19 in Queens, which she describes as most crowded school in city, to offer vote of no confidence in Crew's leadership and to accuse Mayor Rudolph W Giuliani of hiding behind popular Schools Chancellor; photos (M)