Montgomery County prosecutors dropped their charge yesterday against a 30-year-old Poolesville High School teacher who had been accused of having sex with a 14-year-old girl enrolled in one of his classes. The prosecution cited insufficient evidence.

The teacher, John A. Soule, is on administrative leave at full pay pending a hearing on a recommendation from School Superintendent Charles M. Bernardo that he be dismissed because of the alleged incident that prompted the sex charge. The misdemeanor charge of having sex with a person under 15 years of age was brought after the girl's parents hired a private detective to follow Soule, a mathematics instructor. According to police, who arrested Soule Dec. 12, the alleged incident occurred at Soule's Gaithersburg home.

The county prosecutor's office reported its decision against proceeding with the case yesterday to Judge Charles W. Woodward Jr., in General District Court in Rockville, where Soule was to have been tried.

C. J. Vaughey, the attorney who represented Soule, said in an interview later that the charges were dropped because "the girl denied that the [sexual] acts were committed."

The student had been quoted in news accounts as denying that the was sexual activity.

After learning of the prosecutors' action, Soule "was happy," his lawyer said, "no question about that."

"He's a teacher, he feels he's a good teacher and he definitely wants to go on teaching," Vaughey said.

Soule could not be reached for comment. James R. Whattam, an attorney who is handling Soule's case before the School Board said yesterday's dismissal of the charge "certainly won't hurt" that case.

Stephen Rohr, the school system's personnel director, said the dismissal was recommended on grounds of misconduct in office and insubordination.

He said the dismissal of the criminal charge against Soule "does not affect us at all."

In the period before the trial, a lawyer was assigned to the girl to fight her parents' contentions that she and the teacher had sexual relations.

State public defender James McKenna said the lawyer was assigned because "this is an unusual case that required an unusual step."

The girl's lawyer, Camilla McRory said last night neither she nor her client would comment on the dismissal.

Soule has maintained his innocence and described himself as "confident I'll be cleared off all the charges."

Students have described Soule as both magnetic and amiable, and as able to add humor and vitality to everyday educational routine.