As a highschool teacher, I can assure you all it's not as easy as knowing your kids' Internet "friends." So, said pervert or older person poses as a teen, what then? Or, what if your kid encourages an older person to pose as a school friend in order to hide from parents? Are you going to go to science class and demand to meet said Internet friend? I don't think so...
Many teens not only hide info, they manipulate it...constantly. It's not as easy as just understanding your kids - that is showing a lack of understanding as a parent, IMHO. I have teens who are texting drug dealers during the day (we just expelled one the other day). Parents have no clue as dealer poses as a school friend. Due to privacy laws and confidentiality, I can't tell you as the parent if dealer is actually a real person. You are not allowed to come into my school and demand this info. If you do it one too many times, we are then going to call the police on you.
The Internet, coupled with privacy laws, and the insane usage of cell phones by teens, has made a big, bad world that is almost impossible for parents, even the best parents, to keep 100% track of. I am excepting dumb parents in this equation, and I do so a lot of those around - even here on these boards. So many adults seemed obssessed with Facebook, etc., and they are accepting strangers as "friends" or old highschool peers they haven't seen in 20 years! A lot can change in that amount of time...You don't know that person anymore...
So, from where I stand, there are a lot of dumb parents/adults around, but there is also the lure of the Internet and the culture of secrecy that it seems to perpetuate. The Internet is useful for many things, but it's also created a culture of secrecy and deception that is almost impossible to totally penetrate.
Tiger