Trust your kids.
I left home at 16 because my parents were being way too over protective- aka the opposite effect they were hoping for.
Good lord, give your kids some breathing room, especially as they get into their teens! Please, I beg you.
I had so many rules, and even though I was a good kid before the rules, I turned into a bad one after they were put in place.
No matter what you do there comes a time when you can't oversee everything your kids do, no matter how hard you try their going to go behind your back.
I think its probably better to make them feel like they can be open with you and tell you what they're doing - whether that be something you like or not... rather than having all these rules in place that they aren't going to follow as soon as you turn around, and have them lie to you about what they're up to!
The only way to make them feel like they can trust you enough is to give them their privacy and let them come to you.
Monitoring their life and doing things like reading through diaries is not the way to establish a good relationship.
And if your child does open up to you and tell you something you don't want to hear(whether it be that they cheated in school, or that they aren`t a virgin!), DON'T REACT NEGATIVELY. It only reinforces the feeling that they need to lie to you to keep from being in trouble for things that THEY`RE going to DO NO MATTER WHAT. Talk with them about it, work things out in a positive way. BE OPEN.
All you can do is your best in giving them the skills, values and morals they need and then watch them grow up and see what they do with what you've given them.
I'm not saying you shouldn't guide them. I am just saying - restricting them from the world is going to do more bad then good in the long run.
I'm sorry for the rant. My parents were control freaks, and though I know it is because they love me, I still can't forgive them for how miserable they made me when I lived with them.