The alleged incident took place during a school gathering in Dadar area last night, a senior police official said.
"The accused, who apparently works in the school canteen, has been arrested for allegedly sexually assaulting the class V student," he said.
As the victim did not return home, her aunt went to the school and saw her weeping near the toilet. After the girl narrated her ordeal, a complaint was lodged by her family at the Shivaji Park police station, the official added.