The 16-year-old boy is being questioned by detectives after entering Attleborough High School, in Attleborough, Norfolk, armed with a knife, matches and a canister of what was thought to be flammable liquid.
A spokesman for Norfolk Police said they were alerted by a member of the public from Canada who had seen a threatening message posted on the internet.
The spokesman said the informant had no links with the school but had called the force switchboard after becoming concerned.
The boy was arrested as he arrived at school.
Superintendent Katie Elliott, of Norfolk Police, said: "The constabulary was alerted by a member of the public after a message was posted on a website inferring the criminal damage of Attleborough High School.
"When the individual attended the school, the youth's bag was confiscated. Inside, a knife, matches and a canister of what is believed to be a flammable liquid was discovered.
"Our officers immediately arrested the 16-year-old male, who is helping us with our inquiries."
She said all pupils had their bags searched as a precaution but nothing further was found.
Police said the boy's home address was searched following his arrest and a computer was seized and sent for "urgent analysis".