I agree with SalSabeel. Its all on the kid. There are kids in seperated schools that are guys that are attracted to girls in CoEd and NON CoEd schools. My stance is that the more you try to seperate the two sexes, the more they try to interact with the opposite sex, Just like SalSabeel said "in a private school, the more the kids there are forced NOT to talk to the opposite sex, the more they try to do it, whether it be online, outside, whatever" I have also noticed this . In an Islamic Private seperated school, the Priciple's son and the Imam's were caught kissing, and thats a Islamic School!
My dad is a smoker, There are cigerette boxes lying around my house, but I NEVER went and took one or tried one. If my dad hid them, and tryed to put them away from my reach the more I would want to try one. (I am a non smoker)
Another example is like your parents get something, its in a box, you ask what it is, but your parents say its not for you and dont try to peek into the box. You will be dying to open the box and see what it is.
On the other hand if your parents bought something, and told you yeah we bought you a laptop but you cant see it till next week, you will be more patient and will wait the next week to get it.
Its ALL on the Kid. If the kid was raised properly, taught morals and whats right from wrong, there is no reason why they will do anything wrong.