More than 600 students were evacuated yesterday from a Harlem school building after a fire broke out in a coat closet, leaving a classroom ruined and jackets destroyed, but injuring no one. Fire officials said they were still investigating the cause of the fire, which started around 10:45 a.m. in a third-floor social studies classroom and was brought under control within a half-hour. For once, tardiness may have paid off for David de los Santes, 12, who proudly displayed his unscathed down parka as he stood in front of Intermediate School 275. ''The others are wearing Red Cross blankets,'' David said with a grin. ''I was late heading to that class, but look at me now.''