Matthew Musolino, one of three teen-agers charged in connection with the arson of Fort Hunt High School last December, pleaded guilty yesterday in Fairfax Circuit Court to a misdemeanor charge of damaging public property.

Judge Lewis R. Griffith told the 18-year-old Musolino that he could be sentenced to up to a year in jail and fined up to $1,000.

Musolino was allowed to plead guilty to a misdeamenor, rather than face felony charges, because he agreed to cooperate with prosecutors in supplying evidence against the two other youths allegedly involved in the $4.5 million arson, according to Deputy Commonwealth's Attorney Steven A. Merril.

Merril said that because of a lack of evidence in the initial investigation of the arson, his office was forced to promise Musolini that he would only be charged with a misdeamenor in returen for musolino's eyewitness account of the crime.

"We took what we could get," Merril said. He said that he had to choose between prosecuting no one or prosecuting one misdeamenor charge and two felony charges.

The two other teen-agers are Timothy M. Greer, 19, who was found guilty on a felony charge of arson in a jury trial May 30, and Robert Smithwick, 18, who pleaded guilty to arson the same day. All three are scheduled for sentencing July 13.

The jury set Greer's maximum sentence at two years. Smithwick faces between two and 10 years in prison.