A 16-year-old female and two 15-year old-males were arrested this morning in connection with the arson attack at The Castle School sixth form.
An outdoor building which housed two classrooms was set on fire on Monday evening.
The sixth form was closed on Tuesday but reopened this morning.
Headteacher Peter Smart said the fire had left him 'saddened and disappointed'.
The Castle School fire was the latest in a string or arson attacks in the town which led to four sheds at Daggs Allotments being burned to the ground.
It is not clear whether those arrested are also suspected of causing these fires.
A spokeswoman for Avon and Somerset police said: "All three are currently in custody and helping us with our enquiries.
"Anyone who has information that they think might help with our investigation should call 101 and quote reference number 5218278600.
"Parents have been urged to check their children’s social media accounts and contact us if they have any concerns."
For further coverage of the arson attacks go to: bit.ly/2GvdYsi.
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