Rock star Michael Jackson thrilled hundreds of youngsters Tuesday during a lightning visit to Cleveland Elementary School in Stockton, where five students were killed and 30 others were injured Jan. 17 by a drifter armed with an assault rifle. "I think they'll remember this for a long time," said a Stockton Unified School District spokesman. Hundreds of people gathered outside the school hoping to spot the pop superstar, but he was whisked in a rear entrance in a long white limousine and left the same way after about 20 minutes. He visited kindergarten, first-, second- and third-grade classrooms, then dropped in on an assembly of fourth- through sixth-graders. No reporters or photographers were allowed to watch as Jackson met with the youngsters. Said some of the students afterward: "He just smiled, and then he left . . . ." "He just made a peace sign and left right away." Three weeks ago, Patrick Edward Purdy slipped into the schoolyard as children were playing at recess and opened fire with a semiautomatic weapon, fired more than 100 shots, then took his own life.