The remains, thought to date back to the 13th century, were discovered during installation works at a top private school in Scotland.
The bones were buried less than two feet underground in the quad area of the site and are now being examined by experts with a view to being reburied.
Robert Gordon University, in Aberdeen, ordered the boiler work – but the skeletons were discovered in the grounds of the neighbouring private school Robert Gordon College.
The medieval grave contains “about 25 [skeletons] from around the 13th century,” according to the archaeologists.
Contractors installing cables found the first skeleton near Schoolhill main library and called in Aberdeenshire Council archaeologists to carry out further excavations in the area.
It is believed the skeletons are of people laid to rest on the site of the former Blackfriars Abbey during the 13th century.
The abbey was founded between 1230 and 1249, purportedly by Alexander II, but was destroyed by reformers in 1560.