Fifty-one children at the prestigious Beauvoir School in northwest Washington were found yesterday to have head lice, a pest thought until recent years to be confined to the poor.

Frances Borders, principal of the 353-student school running nursery through third grade, said 10 children were sent home. The 41 others remained in school because, "frankly, it was impossible to reach some of the mothers," she said.

The pediculus humanus capitis, as the tiny pest is known to entomologists, is a truly democratic insect, nesting in the hair of urban poor as well as suburban rich.

It is not confined, as was once thought, to filthy heads, being equally at home in a squeaky clean mop.

While there have been some problems this year with lice infesting D.C. public schoolchildren, most of the outbreaks have been confined to schools in upper northwest Washington, which may, in part, have something to do with the racial composition of those schools.

"For some reason the head louse does not appear to do well in the hair of black children," said Dr. Robert Parrott, director of Children's Hospital National Medical Center.

The major infestations this year appear to be among children in private schools. Dr. Bettie Clark, who watches over the health of children in the city's public schools, said "there were a few schools that reported a problem, but a lot of them were private schools. They were calling in to us for information and we told them what our procedure is for handling the situation."

What the District's schools, and most other systems, do is to send home lice-infested children and order them not to return unitl they are lice-free.

Several shampoos will rid a head of lice, including products called Rid. A-200 and Kwell.

The federal Center for Disease Control in Atlanta recommends a second shampoo treatment eight to 10 days after the first, because some eggs may be left after the first treatment.

Authorities stress that the only way to prevent a reinfestation is to wash the affected person's clothing and bedding in very hot water, or have it dry cleaned. Family members should also be checked for lice.

The head louse is basically harmless, albeit obnoxious, creature two to three millimeters long.

The louse's life span is short - only 30 days - but during that time a female louse can lay 50 to 150 eggs, each of which she cements to the host's hair shafts near the root.

The eggs, also known as nits, are a transparent whitish color, somewhat like dandruff.

Despite their unpleasant appearance, head lice do not carry any diseases. They are transmitted by contact with infected clothing, such as hats or scarves, or through upholstered furniture or bedding.