There are good and bad teachers in every school.
Your school was completely different to mine in that case

back in my day there was no such thing as a bad student or a good teacher !
The teacher got all the blame for the poor results, and the student got all the credit for the good ones
I think a lot of parents could do better when it comes to helping their kids with education as it's not just the school's job.
I think this is absolutely key. I am from rural Ireland - my father is a farmer, my mother was a teacher. A large number of my aunts & uncles on both sides went to university and were involved in education in some shape or form - which was unusual given you are talking about 1960's/1970's rural Ireland. Education always was a focus of our youth (mid 80's early 90's), whether we liked it or not, and many a dispute arose when Dad wanted us to help on the farm and Mam wanted us to study. Mam mostly won!
When I compare the results we got to our neighbours/friends, ours were always higher - because of the parental support we received. I knew many a very capable student fall by the wayside because of lack of interest/parents could not care less.
In my view (and experience), an academic child can excel in an average school with the right parental support. Whether an average student can excel in a 'high achiever' school is another matter. The bigger challenge is where everyone around them has no interest, very hard for a single student to make a difference - although I have seen it happen a number of times, again normally with strong parental support and disclipine.
I agree however if 90% of students from school A go to university whilst only 5% go from school B, all things considered, I think I'd know which one I would hedge my bets with!
The school I went to, maybe 10% went to university, 25% went to an RTC, 25% went to the local Ag College and the rest either looked for a job or went on the dole. It was also a small school with maybe 350 students in total in the school.
The challenge for any academic student here was the there was one history/economics/accounting/tech drawing class for both honours & pass, with only Irish/English & Maths split into honours and pass. This means the teacher is trying to bring all students along at a reasonable pace and not losing the academically weaker ones in the class.
The other challenge for the student is having like minded students to interact with, and create a sense of competition between the group.
So yes, the school and interests of the group of friends plays a part in academic success, but in my personal view parents make the major difference. Even the literacy report that came out during the week clearly mentioned the role of parents.
Back to the 90% and 5% - I wonder how many stories the 90% class got read each night before bedtime versus the 5% class? How many of the 90% class were encouraged to make up stories for their parents versus the 5% class. Education starts at a very young age !