There Were three things that characterised an SM in the old days (I mean 1960's).
a) the intake
b) the teachers
c) the ethos
The intake was very broad. Often GS places were thin on the ground and many pupils were sent to SM because of a lack of places not lack of ability.
Notwithstanding those like me who were GS material and were not given the education to which they were best suited (to quote the 1944 education act). There was a whole raft of pupils who what might be known as
"borderline". It was estimated that around 20% of pupils would be sent to the wrong school because the 11+ was quite inefficient (that?s before you took into account middle class tutoring).
That meant that in any cohort in SM at least 10% or more would have IQ's in the top 10& of the population ( i.e. an IQ around 120 - most GS schools accepted IQ ranges from around 115 upwards depending on places available and so they had an equal number of pupils who really were not GS material).
So the intake of an SM was as broad and around the same in distribution as many comprehensives. The top stream (or sets) would be occupied by roughly the same ability range as is seen in most comprehensives today.
What might have been missing were the top IQ's - the 2% but even then many SM had pupils like me who were in the top percentile but for whatever reason were not in GS.
The SM had the same spread of ability across the lower ranges as they do today.
There may have been fewer with SN (or identified SN). SN generally went to special schools back then
But by and large the cohort of an SM mirrored that of a comprehensive.
Most SM schools were inhabited by w/c children. This was very much a fact. Many comprehensives are similar. Some SM were m/c or m/c in them but the overwhelming culture was w/c. This brought with it all the same behavioural and work ethic issues you find today in most schools except the highly selective. The lack of work ethic, school is not cool ethos and the whole idea that classes were for messing about in is not new. Most SM suffered from it. So again many comprehensives mirror that.
The curriculum was geared to the broad ability range, often included lots of metalwork and woodwork (and cookery ) and similar "practical" classes.
The exams were geared to that too - when the pupils took them. The GCSE mirrors more closely the CSE than it does the O level. The B Tech in school was a practical skills course when I was a pupil. So not a lot different there.
Thirdly most SM school were staffed by trained teachers. Many then were not graduates (this has changed to an extent). Many were generalists . many taught subjects not their specialism?s. They were good at crowd control. Many were ?good" teachers ( a la ofsted) but they were not the kind of people that could stretch the most able. Indeed their time was taken meeting the needs of the unruly and the less able so bright kids were left to get on with it - just as in comprehensives now.
Classes were large. I was in a class of over 40 (and that was the A stream!). 30 was a luxury if we got split for any reason (usually boys and girls split for some subjects).
Most GS schools were staffed by the "best" graduates. They had smaller classes. A m/c culture and an expectation. The kind of teachers in GS schools are mostly found in private schools now.
That?s just a little snippet of how things worked.
In effect losing selective schools created a one size comprehensive (SM) fits all school