I went to this school late 90s - early 00s and I am very surprised they still do this.
Quick background, then, forms were labelled C,L,A,P,T,N. Max of 30 in each. From years 7-9, you did all your lessons with your form (mine was C) except Science and Technology were you were randomly mixed with 2 other forms (CLA,PTN) into 3 classes. We assumed this was streamed at first but it wasn't at all.
The only time we were ever 'streamed' was for Maths in Year 10 where we were streamed into 3 sets (we had an intermediate paper, back then). And then the last 4-6 weeks of teaching in Science in Year 11.
The only time "CLA" ever had actual lessons with "PTN" was in Year 10, if you chose the same options and ended up in the same class. Again, not streamed.
Their thought back then was that sets make the "lowest achieving kids feel hopeless". I'm not even lying when I say my HoY said this when streaming was raised at a school council meeting. She also said it encourages us to help each other. That is partly true.
When I was there, behaviour was very bad. It really isn't like that any more. That is why I am surprised they still do this because it was even rare back then. Still, some girls got outstanding results. I got very good results. I'd say on the whole, students excceded expectations. We were only behind Our Ladies (now All Saints) and one or two of the Jewish girls' schools. We also come 2nd in the country for the amount you learn between Year 7 and Year 11 when I was there.