Lessons are not mixed until they get to the 6th form where some general studies are mixed. I recall one year several boys put down a dance module as they understood the girls would be doing it in droves - only to find they weren't
Also in the 6th form there are plenty of opportunities for debating/MuN etc.
There are some other joint activities lower down the school - there are occasional joint appearances in each other's plays & the swimming & athletics teams often compete along-side each other. Every so often there are combined music concerts as well. My daughters have had plenty of chances of doing joint activities but teaching is quite separate & that all seems to work just fine.