Arsonists broke into the Weatherless Elementary School in Southeast Washington before dawn yesterday and set two fires that damaged nine classrooms -- two of them severely, D.C. police reported yesterday.

Principal Shirely McGalliaria said the school would reopen Monday for post-holiday classes, but the nine rooms would not be used immediately. Seven of the rooms suffered only smoke damage and they will probably be ready for use in about a week, she said.

The most severly damaged classrooms were on the ground floor. They will probably be closed indefinitely, the principal said. The same two rooms were reopened only three weeks ago, after being refurbished from damage done last February, when three seperate fires caused more than $200,000 damage to the building.

Yesterday's morning fire, which was discovered by the school engineer when he reported to work about 6 a.m., caused an estimated $50,000 in damage, police said. The school, located at Burns and C streets SE, is down a hill from the Stoddert Terrace public housing project and partially hidden behind Fort Dupont Park, making it possible for fires to go undetected for several hours.

McGalliaria said the school is now without the use of four classrooms altogether, since two of those extensively damaged in last February's fire have never been repaired. o

"I could take it last time but I can't take it this time," she said, adding that less than a month ago vandals broke into the school and set a small fire in the school library.

"I'm very upset . . . It's a lot of work and frustration for people," said McGalliaria, who closed the L-shaped four story brick building for a day to clean up after the February blaze damaged 10 classrooms and left the school yard littered with blackened and scorched debris of books, desks, chairs and papers.

Police said yesterday morning's intruders entered the building by removing a heavy metal guard from a classroom window. They piled papers together from that classroom in front of the doors of two rooms and ignited the papers, police said. These two rooms were the most heavily damaged.

The 10-year-old school is equipped with a security system to detect both intruders and fires, but that system was not functioning yesteday, according to David Huie, director of the school's division of buildings and grounds.