Barbers Hill ISD said students and staff were quickly evacuated from the school when the fire sparked. No injuries were reported.
SkyEye flew over the school, where the transformer appeared to be charred and smoking.
In a later update, the district said the fire was out and students and staff were allowed back inside. However, because of the fire, the power was out at the school, so the district canceled classes for the rest of the day.
Instruction continued at all other Barbers Hill ISD campuses.
Parents were told they could pick their students up as soon as possible.