A FEDERAL judge refused to order an elite Boston public school to admit a 12-year-old white girl while her father challenges a racial quota for students. Allowing Julia McLaughlin to start seventh grade at Boston Latin School when school begins on Sept. 1 could prompt other students to seek reassignment only days before classes begin, US District Judge W. Arthur Garrity ruled this week. Julia's father, attorney Michael McLaughlin, filed a lawsuit earlier this month challenging the constitutionality of Boston Latin's policy of reserving 35 percent of its spots in the seventh grade for blacks and Hispanics. The girl outscored 103 black and Hispanic students who were admitted, her father said. Entrance is supposed to be based on test scores. ''This is an exam school which should be based on merit. Period,'' Mr. McLaughlin said. School headmaster Michael Contompasis argued in an affidavit that ''if we did not set aside a percentage of seats for blacks and Hispanics, we would substantially resegregate Boston Latin School.'' Mr. Garrity, the federal judge who ordered city schools desegregated more than 20 years ago, said any harm Julia might endure by entering classes late should she win the lawsuit would not be as great as the harm to the school if he allowed her to attend classes while the challenge is pending. The judge noted that black students have been rejected at Boston Latin and other schools because seats were reserved for white students. ''Race is a determinative factor right across the board in public schools in the city of Boston ... but it does not follow that race only operates at the disadvantage of whites,'' Garrity said. Court battles over Boston's segregated public schools led to violent conflict over busing after Garrity ordered desegregation in 1974. The order was later vacated in 1987, shifting control of school assignment policy back to the school districts. Julia plans to attend Latin Academy, another Boston exam school, during her court battle.