You could look at whether women who read engineering come disproportionately from schools that were single sex from 11-16. Almost all the women I have met who have read engineering did so. 16-18 doesn't seem to matter (much?), but there appears to be something about the gender influences in 11-16 mixed schools that put women off physics and engineering (and possibly men off English Lit etc). If there is a factor, is it declining?