Matthew Musolino III, 18, who is awaiting sentencing in connection with the arson fire that destoryed Fort Hunt High Scholl last winter, was convicted yesterday of assault and sentenced to 360 days in jail.

Musolino was found guilty by Fairfax General District Count Judge Eugene Morris of assaulting Kurt O'neill, 19, of 2204 Wakefield St., Alexandria, following a patry June 24. O'neill's jaw was broken when Musolino kicked him while seven other youths were holding him on the ground,, according to the prosecution.

O'neill testified that he went to Musolino's home June 24 to demand an apology from Musolino for accusing him earlier in the evening of breaking into Musolino's home. Confronting Musolino and a friend on the street in front of the house, O'niell said he and Musolino argued as the friend went into the house and returned with six other youths.

'neill said one of the group knocked him down and the others started beating him. Then, O'neill said, he heard Musolino say, "Let me do it, let me do it. Hold him," and Musolino kicked him in the face.

Musolino and four defense witnesses denied this, asserting that Musolino was restrained by another youth from touching O'neill. The defense witnesses also said the fight began after O'neill pulled a letter opener on Musolino. O'neill denied carrying any weapon.

Frank Dunham, Musolino's attorney, said he plans to appeal and noted that eight charges have been brought against Musolino since the high school fire last New Year's Eve. Some of those charges, involving such alleged offenses as vandalism, have been dropped. Musolino is scheduled to be tried today on a charge of using "curse and abuse" in an incident involving a barking dog.

Musolino is awaiting sentencing for demaging public property, the misdemeanor to which he pleaded guilty in the high school fire. He could be sentenced to up to a year in jail plus a $1,000 fine.

Morris held Musolino is contempt of court and sentenced him to three hours in jail yesterday for using the work "hell" in his testimony after being warned against using it. Dunham argued that Musolino wasn't using pofanity "to befoul the air," but was only "using his own language."

At the outset of yesterday's 6 1/2 hour trial, Morris denied a defense motion to disqualify himself from trying the case on grounds of personal bias.

In an unrelated case, Morris had revoked Musolino's $5,000 bond and returned him to jail last March in a vandalism case. The charge against Musolino in that case was dropped.