The airline said "emergency fuel" was released to "reduce landing weight," which subsequently fell on five schools in the area.
Mist from the excess fuel caused mild skin and lung irritation to those it fell on which was treated with soap and water, according to officials.
Los Angeles County Fire Department inspector Sky Cornell, said all injuries were minor and there were no evacuations.
"All the fuel evaporated very quickly and nothing flammable remained in the air or on the ground," he added.
Diego Martinez, a pupil at Park Avenue Elementary, said he and his classmates were outside for a physical education class when they saw the plane flying "very close" overhead.
The 12-year-old added the smell of fuel "was very strong".
The school district said in a statement that paramedics were immediately called to treat anyone complaining of "skin irritation or breathing problems".
The pilot was asked whether he wanted to keep the aircraft over the ocean to dump fuel but declined, although it appears he may have changed his mind later about whether the plane could safely land weighing as much as it did.
The Federal Aviation Administration (FAA) said it is investigating and added:
"There are special fuel-dumping procedures for aircraft operating into and out of any major US airport
"These procedures call for fuel to be dumped over designated unpopulated areas, typically at higher altitudes so the fuel atomises and disperses before it reaches the ground."
However, pilots can deviate from the rules in an emergency for safety reasons, said Doug Moss, a retired airline captain.
Delta Air Lines said in a statement that Flight 89 had "experienced an engine issue requiring the aircraft to return to LAX.
"The aircraft landed safely after an emergency fuel release to reduce landing weight".