SAN JOSE -- A one-alarm fire early Monday at a San Jose school, triggered when someone tossed a gallon of Kerosene into a window, has been determined to be arson, school officials said.

The fire at the George Leyva Middle School at 1865 Monrovia Drive, was reported by the alarm company to the fire department at 1:38 a.m. A security guard in the school was alerted to the blaze by a motion detector and reported seeing smoke in the classroom, according to San Jose fire Capt. Alberto Olmos.

School principal Chris Corpus said a suspicious fire had also been set last week in a field behind the school, but there was no indication that the two blazes were connected.

Firefighters arriving at the school saw the fire in a corner of the classroom, Olmos said. The blaze was knocked down at 1:55 a.m.

"It's never a good thing to see damage to something as popular as a school," Corpus said. "There's no reason to burn a school."

Corpus said one other classroom had suffered water damage and that students would use other undamaged classrooms on Monday.

Investigators were working on an estimate of how much damage the fire caused, Olmos said.