internet is a bunch of wires. there is nothing wrong with communicating with friends, learning about a world, and looking up interesting information. Teens should not do inappropriate things online, same way as they should not do them offline. Some teens may be unreliable and should not have access.
At the same time, some may be unreliable in what they do offline. If you worry about what a teen sees in the computer, you should also worry about what he hears from his friends – and even teachers – offline.