About 350 Charles County high school students walked out of classes yesterday and their teachers convened a meeting, both groups demanding explanations for a blackish mold growing in their school that is suspected of causing numerous illnesses.

"People are just coming in not feeling well," said Susan Chalmers, a teacher at Thomas Stone High School in Waldorf. Most of the complaints involve colds, sinus and respiratory infections, headaches, dizziness and tingling in the chest and hands. "I get the feeling {school officials} know things they haven't told us," she said, explaining yesterday's actions.

Linda Dent-Brown, a spokeswoman for Superintendent John H. Bloom, said yesterday that tests conducted in September by an environmental firm showed high levels of a typical outdoor fungus in the library. The fungus, aspergillas niger, is not considered dangerous, Dent-Brown said. But in a letter sent home last week, parents of the 1,440 students were advised that the fungus may exacerbate allergies and respiratory problems.

Gene Bridgette, vice principal of the school, said absences and reports of flulike ailments had been higher than normal.

In the meantime, the results of tests conducted elsewhere in the school are due back Dec. 15, she said. Maintenance officials and other school staff members are scheduled to tour the entire building today to look for other signs of the fungus.

"I'm telling {parents, teachers and students} . . . everything that I have so far," Dent-Brown said.

Aspergillas niger is among the common microbes that contaminate air and has been cited in numerous cases involving indoor air pollution. Often its presence is felt by people in buildings with poor ventilation.

Last year, two classrooms in a Fairfax County school were boarded closed after teachers became ill. Cited as a likely cause was the "sick building syndrome," an increasing problem for well-insulated, energy-efficient buildings that allow little outdoor air inside.

Thomas Stone, a sprawling two-level building built in 1969, has few windows, and teachers complained yesterday that classrooms have little ventilation. Gray and black mold can be seen on classroom walls, they said.

The walkout, which ended an hour after it began with most students returning to class, was an illustration of concerns that the staff and students have expressed since last year, they said. That's when the numbers of puzzling illnesses were first noted, Chalmers said.

"I've refused to teach in my classroom," public speaking teacher Michelle Simpson said yesterday. Her room and 13 others are on an underground level where many teachers and students have become ill, she said. Since September, she said she has had an enduring cold, and both a sinus and respiratory infection. "I've moved to the auditorium," she said.

The library has been boarded up since September, its drapes and carpet replaced, as were some books. It was disinfected twice, and is to be opened this week, Dent-Brown said. The cost of the cleanup was estimated at $100,000.

One staff member has been transferred to another school because of illness associated with the building, Dent-Brown added. Based on advice from the county health department, there are no plans to close the school, she said.