An employe of a computer firm giving lessons at Nativity Elementary School in Northwest Washington was charged yesterday with performing an indecent act on a 9-year-old girl at the school, D.C. police reported.

The instructor, Gary Nuhn, 30, of 9207 Cedar Way, Bethesda, was arrested yesterday morning after the student complained that the instructor had put his hand under her dress while she sat at a computer terminal.

Nuhn was employed by Compu-tech Educational Services Inc., of College Park, and had given lessons at Nativity once a week since late October, according to Maeola Lavergne, principal of the Catholic elementary school at 6008 Georgia Ave. NW. She said the school made arrangements for the lessons but that parents paid tuition directly to the firm.

Lavergne said the incident was reported shortly after Nuhn gave his last lesson in mid-December, shortly before the Christmas break. She said the firm already had decided to have another instructor continue the lessons this month.

According to police, after Nuhn put his hand under the girl's dress, he asked if she "liked it." Police said he stopped further contact when the girl said "no," but that he acknowledged later that the incident took place and said he needed psychiatric help.