Hello Hans, Sorry to be so long in giving a reply. This thread missed my attention.
You are more or less correct. Colwyn Bay County School was the one kids went to if they passed some sort of exam aged about 11. Around about 1946 we were all called into the main hall and Sam Dodds, the headmaster, said "as from this day you must never call it the County School. It is now Colwyn Bay Grammar School." Later on it opted out of government control and renamed itself Eirias High. So, it WAS a secondary school---but it was never officially called that.
Now to the Central School. That was on the other side of the Dingle. Thats were you went if you failed the exam. Later it was renamed the Modern School. I'm not being snobbish here, it was a very good school in more practical skills. No harm in that, its not everyone who is good at Latin prose.
Just for a laugh I will mention that very recently, around four or five years ago, I was working for a firm in Llanrwst. The front of house man, the man on the reception desk, answering the phone, doing all the odd jobs. By the name of Raymond Ellis, from Colwyn Bay. He was prone to boasting "I was the HEAD boy in Colwyn Bay Modern School." I was just a driver there, but one day he had annoyed me even more than usual so I replied "So---you were the head boy. Well I was the worst pupil in Colwyn Bay Grammar. But the worst in the Grammar is better than the best in the Modern, because "I" passed the exam. Lots of laughter all round Mike