Preswylfa School was at 82 Pen-y-lan Road, on the corner of Pen-y-Lan Road and Sandringham Road, Roath Park, a building still there to this day.
The first mention in the newspapers of the school appears to be 1910 when it was advertised as Preswylfa High School, a good Day and Boarding School for Girls. Roath Park Terminus. Principal: Miss Jones. They had a second premises at 2 Newport Road (presumably catering for those who had missed the tram).
The booklet that recently appeared on e-Bay is of unknown year but probably soon after the school opened as it has Miss Jones as Principal. The Telephone number was 28 indicating their early take up of the telephone service. I wonder how many calls they received? The school included a kindergarten and also took boys up to the age of 10. Day boarders were charged at 6½ Guineas a term and a terms notice was required if you wanted to withdraw your child.
In 1917 Preswylfa School had a second premises at 87 Ninian Road.
The curriculum was broad offering English Education, Religious Knowledge, Mathematics, Science, French, Latin, Music and Singing, Drill and Dancing, Plain and Fancy Needlework, Handwork, Swimming and Sports.
After all that education the pupils were ready to sit entrance examinations for Oxford, Cambridge , London and Welsh Universities, College of Preceptors (teacher training), Royal College of Music, Royal Drawing Society, Elocution, Shorthand, Royal College of Arts, Book keeping and Commercial Practice. Later newspaper adverts included typewriter training.
A notice from 1929 tells us that the colour of the hatbands has changed and will in future be green in colour with a white monogram.
In September 1939 the school was advertising vacancies for pupils unable to return to their own school and boasting a specially constructed ARP Shelter.
The school ran up until the end of WWII. Sometime after the war it was turned into the Penylan Hotel. The building these days has yet another use and is the Penylan Residential Hotel Care Home.
References: Thanks to Ian Clarke on Cardiff Now & Then Facebook Group for posting the pictures from eBay and Find My Past for the Nwspaper Cuttings.
Roath Local History Society 
I notice ‘Preswylfa High School’ is written in white letters on the brick wall (left of photo) that still exists, resting on a steel beam spanning over the Nant (Roath Brook). If the school owned that wall (it was able to have its name on it), could it explain why there was (and still is) a footbridge behind the wall, from no.82 to no.84 Penylan Road? Could no.84 (today Agile Physiotherapy) have been additional residential accommodation? I’ve always wondered why there was a footbridge there – nobody occupying the buildings seems to know.