Worrall's Directory of South Wales 1875
SCHOOLS
Industrial School: Glenffrwd Cottage, Mountain Ash, Miss Barnfill, Mistress
Only reference I can find in Historical Directories (on line) -
http://www.historicaldirectories.org/hd/(note - listed as Glenffrwd NOT Glanffrwd)
The following advert appeared in the Western Mail, Aug 14 1876
TO BUILDERS AND CONTRACTORS
Merthyr Tydfil Union
The Guardians desire TENDERS for making ALTERATIONS of the ABERDARE HOSPITAL for converting same into an Industrial School. Plans, Specifications and Form of Tender to be seen at the Workhouse Merthyr, on and after MONDAY 21st inst. Sureties required. Tenders to be sent in by 10 a.m Saturday, 2nd September next marked "Tenders for Converting Aberdare Hospital into an Industrial School"
By Order, Frank James, Clerk, Workhouse Office, Merthyr, 12 August 1876.
Kelly's Directory 1895 lists a "Probert, W, Glanffrwd, Mountain Ash" and also the Industrial School at Trecynon, Hirwain Rd. It's likely that, once built, the new Industrial School replaced the Glanffrwd Cottage school.
You can search these Directories on line - see link above.
regards