I am so so sad to hear what a tough time your kids are having in and out of school. Unfortunately schools have their hands tied so often and then by the time they should take action there is a slack deputy or head teacher who wasn't paying attention from the start and now finds it's totally out of control.
The boss at the high school next door to where I work suspended some boys last year and this stopped them being able to play on the footy team. They decided to vandalize his car....unfortunately my car looks just like his and it got attacked!
These kids were seen doing this and heard discussing it. I contacted police and basically it was "bad luck to me".
I believe due to some stupid teachers abusing their power teachers have now had so much power removed that they can't or aren't game to take real action. Often also the parents of those horrible bully kids bully the school too!
This is absolutely no excuse at all for the horrible behaviour of bullys that your children have been experiencing.
I think the experiences you are having are the outcome of schools having less power. It is because parents won't believe teachers when they say "Your child is being a bully" and back up the school. Instead we stupidly blame the victims....it's getting worse not better......
All I can say is stand up for your rights. See if you can find a social worker who is willing to go into the school and report the behaviour as it's happening.
Above all protect your precious children