Social networking, it can be used anytime. Plus, you cannot ban it effectively. Besides, there are ways to bypass school filters. The most obvious is using a phone. Since a phone does not connect to the school internet, it can be usable. Also, proxies can also work, and very tech savvy people can bypass it themselves.
I don't think that schools should ban social networking. Social networking can be beneficial in many ways. Students can use to connect to other like them. For example facebook now days have clubs for all kind of activities like University Club, features like these can help students. Social network has real become the way of living.
Schools can prevent teens from social networking while on school campus, but it is not up to them to decide how students present themselves online outside of school. The parents of the teens can decide for temselves whether they want to keep the students from having an online presence or not.