The fire, which burned in a double-wide trailer behind Belle Haven Elementary School at 415 Ivy Drive, was reported by a neighbor at 8:49 a.m., Menlo Park fire Chief Harold Schapelhouman said.
Firefighters had the one-alarm blaze under control by 9 a.m., and no one was injured.
The trailer is one of several portable classrooms in the rear of the campus, and is used for preschool and as an office space for an after-school program, Schapelhouman said.
School was not in session, so no children were in danger, he said.
The trailer and its contents, which included computers and other office equipment, were seriously damaged and will most likely not be usable.
"If the trailer can't be repaired -- and we don't believe it can -- replacement is an option, preferably before school starts," he said.
He said school officials estimated the fire caused about $150,000 to $200,000 in damage.
The cause of the fire is under investigation, and Schapelhouman said it was most likely accidental.