A mother who lifted her dress as a show of defiance against her son's algebra teacher should not have been convicted of lewdness, three state appeals court judges have decided.

In a decision released Monday, the judges reversed the conviction of Marie Serpente because she was wearing underwear beneath her dress. Serpente had been convicted of lewdness involving a child, a Class-A misdemeanor.Serpente went to her son's algebra class on April 4, 1986, to protest her son's being kept after school two days earlier, according to the court's decision. The boy had fallen while walking home in the dark and had injured his ankle.

To protest the detention, Serpente entered the algebra class while in session and began accusing the teacher of being irresponsible to her son. When the teacher asked her to leave, Serpente lifted the back of her dress above her buttocks and said, "To you, sir."

The name of the school was not included in the court's decision, signed by judges Judith Billings, Gregory Orme and Norman Jackson.

Serpente was arrested after several other confrontations with school officials. In addition to the lewdness charge, Serpente was charged with assaulting a police officer, disorderly conduct and committing unlawful acts in a school.

She was convicted of all the charges but is contesting only the lewdness count. The trial judge had denied a request from Serpente's attorney to dismiss the charge.

The judges noted that none of the students who testified at the trial said they saw Serpente's buttocks. They note the state has urged them to adopt a more liberal definition of indecent exposure, making it illegal merely to attract public attention to one's private parts.

"This we are unwilling to do," Billings wrote. "Given contemporary fashion designs and revealing swimwear, the state's definition might well subject many unsuspecting citizens to criminal prosecution for unlawful exposure."