Seven of the injured were children on a field trip to the Cable Car Museum, The San Francisco Chronicle reported Wednesday.
Some 31 children from the Vision Academy Day Camp were aboard a bus sitting at a stop Wednesday morning when it was rear-ended by a trolley traveling less than 1 mph, said Paul Rose, a spokesman for Muni, the transportation authority.
The children ranged in age from kindergarten to eighth grade.
The 24 students who were not injured were given a free tour of the International Art Museum of America before being taken on a second bus to the cable car museum.
Cause of the crash was under investigation. Both drivers will be given drug and alcohol tests, as is routine, Rose said.