Students were in the lab moments earlier, but were at lunch when the fire started at about noon, Hoff said.
School officials learned about the fire when a fire alarm went off, Hoff said.
After determining where the fire was, Assistant Principal Rick Jenkins looked into the classroom through a window and saw flames coming from the heater, Hoff said.
The heater is a self-contained unit similar to heaters for individual rooms in a motel, school officials said.
Jenkins and Assistant Principal Paul Hercules grabbed fire extinguishers and went outside and sprayed the extinguishers into an outside vent on the heater.
That seemed to reduce the fire significantly, Hercules said.
Firefighters arrived and continued extinguishing the fire in the area and a sprinkler system in the room went off, Hoff said.
Because the computers were hit with water, it is believed that all 30 computers in the lab may have been destroyed, Hoff said.
Students were dismissed early at about 1:30 p.m. School officials decided to dismiss students early for several reasons, including that most of the day was over at the school, Hoff said.
The computers recently were installed in the lab and were used for a graphic design class, research projects and other uses, Hoff said.
School officials were hoping they could install some graphic design programs on other computers in the school so students can continue their work.
"We'll make it work. It's just an inconvenience at this point for our kids," said Hoff, who added that classes are scheduled to resume at their regular time today.
No one was injured as a result of the fire and there was little smoke in the school, Hoff said.
The lab is one of three computer labs in the school.
A $15.3 million renovation of the school is under way and the project is expected to be completed by the fall of next year.
In recent years, problems started to surface at the school, including deteriorating wooden doors and torn and broken seats in the auditorium. The school opened with 900 students and today has about 1,600 students. At times, the school has been about 200 students over capacity.