I'm a school teacher and I would never let my class behave in this way. Please don't generalise like this, I'm actually offended!! As I'm sure some parents on this thread are too!
As I stated earlier, I'm a father of three and I'm not offended because my kids can (and do) behave.
Mazfc, if you keep control of your class then kudos to you, unfortunately I too have seen teachers turn a blind eye on many, many occasions. On an end of year outing to Twycross about 15 years ago my own teachers were noticeably absent when it came to keeping an eye on their charges. It was so bad that me and a friend were not allowed in the reptile/tropical house because of earlier actions of certain classmates.