A firework damaged part of the school, with emergency services spending last night putting out the fire.
The letter read: "We are sorry to hear that your first day at the big school has been ruined by a very badly behaved person who set off a firework that damaged a part of your building.
"It was a very busy night for everybody as the fire brigade came rushing over to put out the fire.
"We also sent out some police officers to make sure the area was kept safe for the fire officers to do their work. We all had lots of other people to look after and keep safe but because of the fire at your school we had to leave them until the fire was put out.
"I hope that nobody else got hurt while we were kept busy dealing with a fire that would not have happened if people didn't play with fireworks.
"I would like to reassure you that we have started an investigation and will try to find the people responsible. If we find them, they will be in a lot of trouble.
"We will arrange for Police Officers to be on patrol in the areas around your school to try and prevent this happening again. Please ask your parents/guardians to call the police if they see people playing with fireworks so we can send officers out if they are not already in the area.
"APS 5767 McFarlane
"Aylesbury Neighbourhood Policing Team."