When firefighters arrived at the school in the Pico-Union district around 3 p.m., they were greeted by "a frantic teenager outside the closed school, stating his friend inside needed help," said Erik Scott of the Los Angeles Fire Department.
The school was closed, so firefighters used a power rotary saw to force entrance through a door, then ran down a hall to an outside pool where two other teenage boys were located, Scott said.
The pool was surrounded by a tall fence, which the firefighters jumped, also throwing their gear over to help the teenager.
Firefighters began immediately to provide advanced medical aid to the 16- year-old boy and rushed him to a hospital in critical condition.
"Fortunately, due to aggressive efforts, the patient was showing signs of improvement," Scott said.
Scott said the circumstances by which the teenagers got into the school are not known, but he said the situation clearly showed the need for adult supervision at a swimming pool.
School and city police are talking to the teenagers and their parents in an attempt to find out what happened.
Citing information from police, Fox11 reported that the three boys were playing soccer near the school, their ball went over the fence into the pool, they went into the pool after the ball and wound up swimming.