Why do so many adults have problems with kids getting on myspace while at school?

I don't want them on the site even when they are at home much less at school unless they are somewhat supervised. I want to know the information that they are putting out there to the public. To me..it really is about keeping my teenager safe. I realize that the whole school day is not used on studying. Of course children are also going to have fun and socialize. That is all a part of growing up. I want my children to have the whole experience of school but in the meantime....it is my responsibility as a parent to protect them.