The buses were parked at the school during the weekend
Two schools have been damaged in separate arson attacks.
An English classroom at Lismore Comprehensive, Craigavon, County Armagh, was destroyed in a blaze at 1930 GMT on Sunday.
It is estimated that the damage will cost £30,000 to repair.
A spokesman for the Fire Service said there were signs of a forced entry at the school.
School principal Joe Corrigan said: "It is so needless, destructive and so unnecessary for people to do that because we have a lively and vibrant community here in Craigavon.
"The school is a hotbed of activity."
Mr Corrigan said he did not believe the attack on his school, a Catholic comprehensive, was sectarian.
"I do not for one minute believe it was a sectarian attack.
"Unfortunately, I think it is down to a very small number of people who believe in vandalising their communities rather than building them up," he said.
Nearly 500 pupils in Years 8, 9 and 10 at the school were asked to stay at home as part of the school was closed off and a clear-up operation began.
Meanwhile, in Newtownards, County Down, a minibus was destroyed in an arson attack at Regent House School.
Three buses were set alight at 0530 GMT on Monday. One was destroyed and two others were damaged.
The front of the school building was also damaged.
Police in Newtownards have appealed for witnesses to contact them.
Northern Ireland Office Minister Jane Kennedy said she was "appalled and saddened" by the attacks.
"These incidents are shameful and those who carried them out have nothing to offer society," she said.
"Schools should be a haven for all our young people and a place where they can concentrate on their education. These attacks serve no other purpose than to divert resources away from the classroom.
"Everyone in a position of responsibility in our society needs to make it clear that these attacks are totally unacceptable."