It happened around 4:00 Sunday evening on the 600 block of Martin Terrace.
Officials say the fire wasn't inside the school building but was contained to two trailers behind the school.
The school had recently obtained the trailers but they were not yet in use.
No one was injured but the mobile classroom sustained damage.
The fire was brought under control by about 5:30 p.m.
No word on a possible cause.