I think some kids of that age might find a uni sex toilet intimidating, and possibly put off going to the toilet altogether which wouldn't be good for them. If it's a senior school then there would be an age range from 11 to 16, possibly to 18 if it has a sixth form, an 11 year old girl might not be too happy to find some strapping great boy outside the loo, or visa versa, small boy and a gaggle or older girls. Personally, I wouldn't welcome uni sex toilets at my age, and think the only occasion where we are all forced to use the same toilet is on a plane.