Ashley Montes, 12, and Timothy Reed, 14, were last seen boarding a bus en route to Venice Beach about 4 p.m. near Mark Twain Middle School in Venice, according to LAPD Officer Mike Lopez.
Earlier in the day, the pair swam in a pool at the middle school, where they are enrolled, Lopez said.
Each has a cellphone, but neither has responded to calls, Lopez said.
There's no sign of foul play, police said. But parents and family members were concerned, Lopez said. The parents of each child reported them missing about 11 p.m. Saturday.
Police described Montes as Hispanic with black hair and brown eyes. She is 4 feet 8 and weighs about 80 pounds.
Reed was described as Latino with brown hair and brown eyes. He stands about 5 feet 7 and weighs about 150 pounds.
Anyone with information about the pair is asked to contact the LAPD's Pacific Division at (310) 482-6334.