Because they know how to work their parent to get a reaction and get them marching into school to "defend" them.
It is ridiculously common for the bully to play victim. It is virtually non-existent for an entire class to gang up on 1 child. It is vanishingly rare for not 1 of those 20 to stand up for what is right, or at the very least, encourage others to 'leave it'. However, it is common, for children to have had enough of a bully by Y5/Y6 and to say things back.
How would your younger child witness what went on if they were in another class?
Do you ever question your children when they tell you this, that and the other has happened or do you believe them verbatim?
The more you post, the easier it is to see your attitude towards this. 2 separate schools, and multiple teachers, and support staff, and 20 odd kids are not the problem here.
I posted about a similar experience I've been privvy too up thread. The worst thing about it is that the poor kid hasn't got a hope in life. He's gone through how ever many years of life getting his own way, thinking he's untouchable because mummy and daddy will come and defend him, but when he gets older...that's not going to happen. If he does what he does in school to people on the street, god knows what the consequences could be. If he carries on into adulthood, he'll never be able to keep a job.
You've asked for advice, take it.