Students at William Byrd High School are being tested for unusual symptoms that students describe as involuntary twitching.

Roanoke County school officials would not comment on the symptoms or say how many people were affected, citing privacy laws. But students interviewed Friday said about three students and one teacher have been twitching their arms uncontrollably over the past few weeks.

"I have seen them in the hallway, and it seems pretty bad," said Charlie Wallace, a senior. "It's uncontrollable. Twitching, that's the only way you can describe it."

The teacher started getting sick last year and had to take medical leave, he said.

"She's not fine," he said.

People affected by the mysterious symptoms could not be reached Friday.

School spokesman Chuck Lionberger said those affected are not dealing with "common symptoms."

"We're dealing with a medical mystery," he said. "The type of symptoms that are being displayed are unusual in their nature."

The school was not closed Friday, and Lionberger said he expects the school will reopen as usual Monday. Lionberger would not say whether those affected were at school Friday or whether they were hospitalized.

The school system sent a letter home to parents Friday about the situation.

"There is no evidence that the condition is contagious and no evidence of any apparent public health hazard," the letter said.

Lionberger said the school system has known about the problem for "a little bit" and has conducted environmental tests at the school. He refused to be more specific about the chronology, but said the problem became more "prominent" over the past couple of days, prompting the school to draft the letter to parents.

Friday's disclosures have worried students and their parents, some of whom picked their children up early.

"They're freaked out," Wallace said, describing his classmates.

"We are very concerned," said Deanna Janowicz, the mother of a William Byrd student. "We are hoping they are coming out with something soon so they [students] can go to school safe."

Janowicz's daughter, Erica, a senior, did not leave school early Friday. She said a friend of hers is one of the afflicted students. Besides twitching, her friend has also been suffering from headaches, Erica Janowicz said.

"She tries to be as normal as she can. It's sad that this has happened to her," she said. "It started with headaches. She has really bad migraines. She was just really tired and started getting sick."

The school system is cooperating with the Virginia Department of Health, but health officials have so far not been able to shed any light on the problem.

"It's a big unknown for us, too," said Robert Parker, a health department spokesman.

"We want parents to let us know if there's anything with their students out of the ordinary," school board member Mike Stovall said.