UPDATE: 9.26am
A statement on the school website said there had been a small fire on the first floor of the science block.
The fire service was called at 7pm and said crews were on site "within minutes".
They were able to put the fire out quickly and without incident.
They added the building was empty and no-one had been hurt.
The cause of the fire is as yet unknown.
The school said they are working on an alternative timetable to ensure any disruptions to science lessons are kept to an absolute minimum. Pupils, parents and staff will be kept fully updated.
UPDATE: 7.41am
FIREFIGHTERS from South Wales, Herefordshire and Gloucestershire tackled a blaze at a school in Monmouthshire.
The fire at a Monmouth School is reported to have broken out in a detached science block last night.
No-one is reported to have been hurt and an investigation is set to take place today.
Assisted @SWFireandRescue with a room fire in school, Monmouth. FIRE extinguished by crews. @Glosfire @gwentpolice also assisted. #FRS
— HWFire Whitchurch (@HWFireWchurch) September 20, 2016
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