Walt Whitman High in Bethesda is now the center of an arson investigation.
Authorities say someone threw an incendiary device into a classroom late Friday night, causing a small explosion and fire.
That set off the sprinkler system, causing water damage to several classrooms on the first and second floors.
"We're not certain of the device," said Assistant Chief Scott Graham of the Montgomery County Fire Department. "It is a crime."
Authorities say the incident happened just before midnight Friday. The device was thrown into a classroom around the back of the building on Whittier Boulevard. Damage is estimated at $50,000.
"Certainly whoever did this has to realize they could have caused some serious damage," said Dana Tofig, a spokesman with Montgomery County Schools. "Someone could have been hurt. Whoever did this will face very serious consequences."
Authorities say there are security cameras positioned in and around the school. Tape is being reviewed as investigators try to find whoever is responsible for the arson.
Crews have been working all weekend to clean up the damage. They pumped out water and used high-power industrial fans to dry out the classrooms.
School is expected to open on time Monday with no disruptions.