I went to my Secondary Modern, in September 1964, as, like 80% of all children, I had failed the 11+.
As it turned out, I went to a very good SM school. But we were not able to learn any languages, in the same way that the Grammar did a few miles away, as we didn't have a language Lab. Only the top stream in my school was able to learn French at all.
Science was taught in just the one classroom, unlike the Grammar, where they had separate laboratories for Physics, Biology and Chemistry.
When I was 15, we moved and I went to a Comp. which had everything that was missing from my old SM.
So, that is my experience. There is no doubt which was the better school at age 11, but, as I said, the Teachers at the SM did their best under the circumstance. The best Teachers, of course, all taught a few miles away at the Grammar.