I can see how his parents should have took him out, but devil's advocate it's not really that easy to relocate your kids. There could be literally hundreds of miles between schools he could attend. Private schooling costs a lot of money (even hundreds per month) and kids can be very shitty there too. Homeschooling requires at least one parent to stay at home, the curriculum costs money, and you have to make sure the child is up to date with everything. Not every parent can afford to do this, sadly. I have no idea what the laws are in their place, so they could have definitely been forced to keep him to school no matter what. Its not always as simple as taking your kid out, especially since police can and will be sent to your house if your kid is not attending school.
So, while they should have absolutely looked into removing him from the situation I'll also say that doesn't solve the problem of shitty kids, their shitty parents, and school faculty (not the teachers, their 'superiors') The school itself has way more resources, possibly, at its disposal and parents should be forced to actually try and parent their problem. Not to mention there are probably plenty of other kids being abused by the bullies that their parents don't care or simply cannot afford to change schools.
I don't like the fact that this happened, at all, and I would want change. The real answer can be everyone in this situation has a hand in what happened but that doesn't mean we should ignore the real perpetrators and enablers, either.