Teachers quite rightly complain about how pupils cannot sit still and keep quiet in class. The official reason for this is that their lessons are not sufficiently interesting and exciting..
The real reasons are:
1) Some have seen through the bluffing and realised that they can do pretty much what they like.
2) Fed on a diet of sugary drinks and snacks throughout the day, many kids are in a constantly hyperactive state. To excuse this, a whole load of medical problems have been dreamt up by drug companies eager to sell new products to the gullible. Every class register is now chock full of acronyms and excuses such as ADD, Bipolar Somethings, Aspbergers, Oppositional Defiance Nonsense and of course ADHD. Here's a handy tip: if your own child is naughty just send a letter to his form teacher with your own impressive sounding disease and his every misdemeanor will be forever excused. It's like the 'Get Out Of Jail Free' card in Monopoly.
3) If I am cooped up all day without taking any exercise I will cause mayhem and go mad. It's no suprise that the kids do exactly the same. Many are driven to school and simply forget their kit or waste their time playing table tennis in PE. Nobody dares force them out to do physical activity and so they burn off their excess energy in your lessons.
Ask anyone who has taken a school trip somewhere involving lots of fresh air and physical activity, whether there were any problems from the naughty kids after the first couple of days.
Private Schools and institutions such as the Armed Forces have known this for centuries and make them do sport every day whether they want to or not.