Crews rushed to the Claughton Centre, on Blowers Green Road, after reports of smoke coming from the building.
The blaze involved a large pile of timber and carpets and was extinguished after firefighters arrived at around 3pm on Saturday.
A spokesman for West Midlands Fire Service said: "Both crews at Dudley mobilised today by [Staffs and West Midlands Fire Control] to report of smoke issuing from the old Claughton Centre, Blowers Green Road, Dudley.
"Large pile of timber and carpets on fire which is now out. Please don’t play with fire or have bonfires near any buildings."
The school was previously ravaged in an arson attack in July last year, which left it without most of its roof.
Last month council planners gave the permission for the site to be converted into 30 flats, made up of 19 two-bedroom apartments, seven one-bedroom homes and four studio flats.
The building dates back more than a century, first opening up in 1904 as the Dudley Upper Standards School for children aged 12-18.
It underwent a number of name changes in subsequent years before becoming the Sir Gilbert Claughton Grammar Technical School in 1957 – named after the Worcestershire-born 19th century businessman and politician.
It operated until 1990 when the school closed and the building was later used as offices by the local authority as the Claughton Centre. It has been unused for many years and the centre was sold for £405,000 at an auction in May 2019 – more than double the original asking price of £200,000.
Previous schemes put forward to build new houses on the site or convert the building into flats have not come to fruition.