Murdering at the school

The siege

Before sieging the school, the cops hided in some bushes to define their positions. Frank was the leader. It was night time.
  "Charles, you stay here leading the siege."- said Frank- "That group over there will form the siege, and this small group will snick into the school. The headmaster sleeps in his office. The guards must be already advised for some cops. Let's go!"
The group leaded by Frank entered the school. They got down the long ramp and found the buildings. They entered the one at the right and got in the front of the headmaster's office door.
"3"- said Frank- "2, 1"
"IT,S OVER, PINCHER"- shouted Frank, but for everyone's surprise, he was not there.
"Damn, he spotted us and ran away!"- said Frank- "You and you are going to the canteen, you and you go check the place where the dead body was found and you, you and me check the buildings"
  It passed one hour while Frank was talking to Charles in the Walkie-Talkies. Frank decided to go check the groups.
When he reached the canteen, he heard someone playing the piano(there was a office in the canteen with a piano). He entered the canteen and said:
"Cops are not supposed to play the piano!"
The sound stopped.
But then he looked shocked to a table that was with some candles. There was something else on that table. There were three decapitated heads on the table. Two of them were the cops he sent to there, and the other one...
It was headmaster's Pincher head.
Frank quickly turned the walkie-talkie and said:
"Charles, the killer killed two cops and headmaster Pincher. Take all the men on the entrance and seize the canteen. He's here.
When the cops seized the building he entered the building and felt like the stupid man alive. There was an open window in the office.
"The killer got out of the canteen when I entered and ran away by the entrance when there was no one there."- thought him
They looked for all the school and it's surroundings, but didn't found him.
They met all at the school entrance and had a "reunion".
When they were entering on the cars to go away, Frank said to Charles:
"Charles, I think I know who is the killer."
"Tell me Frank. Tell me who was the bastard that killed my son"- said Charles
"It was Kyle"- answered Frank- "The bastard who killed your son is my son."