At least 17 children and several adults are being treated for skin irritation and breathing problems.
Fuel may be dumped in emergencies landings, but only over designated areas and at a high altitude, aviation rules stipulate.
The Delta Airlines flight returned to the airport due to an engine issue.
All of the injuries at Park Elementary School in Cudahy, some 16 miles (26km) east of the airport, are said to be minor.
According to the Los Angeles Times, two classes were outside at the time the fuel was released.