A 33-year-old former elementary school teacher pleaded guilty yesterday in Montgomery County Circuit Court to a single criminal charge stemming from her months-long sexual relationship with a 12-year-old boy whom she had taught in the third grade.

Kathleen Gladner, of 9221 Frostburg Way, Gaithersburg, sat in court for nearly two hours yesterday, her shoulders slouched and head downcast, as her lawyer and psychiatrist described her as a troubled, "child-like" woman whose "only emotional resource" during the breakup of her marriage was the boy, now 13.

"With this young man, there was a sense of acceptance and no need for pretense. This relationship was special for her," said Robert Quinlan, her attorney.

He said that the boy began baby-sitting Gladner's daughter, then an infant, four years ago and when Gladner's marriage began to crumble, she turned to the boy "as friend and confidant."

Then, after two affairs with men her own age failed, Gladner in the spring of 1984 seduced the boy, Quinlan said.

The relationship continued at Gladner's Montgomery Village home until January, when the boy's parents found love letters from Gladner and pressed charges.

Gladner pleaded guilty to a third-degree sex offense, a sex act with someone 15 years old or younger. Conviction carries a maximum prison sentence of 10 years.

But neither the boy's parents nor school authorities sought imprisonment for Gladner, who resigned as a second and third grade teacher at Stedwick Elementary School in Gaithersburg shortly after her arrest, according to Andrew L. Sonner, state's attorney for Montgomery County. The boy's name is not being used in this story to protect his identity.

Under her plea agreement, Gladner agreed to stay away from the boy and to continue seeing a psychiatrist.

"As horrible as this whole thing has been, there has been some good that's come out of it," Gladner told the court yesterday before Judge David Cahoon imposed sentence. In an even, but barely audible, voice, Gladner said the experience has "jarred" her.

Gladner told Cahoon: "I can't change where I've been but hopefully, I can change where I'm going for me and my daughter."

Cahoon, noting that Gladner's "criminal act . . . violated a very significant trust that was placed on her by virtue of her employment," sentenced her to three years supervised probation. He ordered her to pay court costs, perform 150 hours of community service and "not become engaged in any occupation or volunteer work" involving juveniles.