Oh freckle, I thought I hadn't seen you around much on mumsnet during the last couple of weeks and now I can see why!
I never realised your son had such a problem with bullying. I bet you are extremelyu relieved it is so near the end of his time at primary school.
I think you are doing the right thing for now in taking him out of school. If your school is anything like ours, I gather the Y6 children won't be doing much actual 3R's work between now and the end of term. Now the SATS are over, it's art classes and PE every afternoon. Your son has done all his exams, so no stress about missing important work.
I am sure ther must be a solution to this de-registering problem. I just cannot see how the grammar school would refuse your son his place if you move him out of primary school for the reasons you state. I am so cross no one was able to reassure you when you asked - I bet you are worried sick! Surely if your so is still with the same LEA, it doesn't matter what primary school he is officially going to. Anyway, what do I know - what you need is official reassurance and get the answer in writing.
I wonder if there's any chance your son could sit in on a few lessons at his grammar school? probably not, but that might be one possiblity.
I have to say, I think the teacher was so out of order in agreeing with the bullies, and I hope he or she realises that was the final straw for you. Later, when you have the energy, I think you should take this further - look at the official complaints procedure.
Not so serious as your situation, but my y6 son has suffered some bullying especially this past year. He has been 'accidently' hit or tripped over, with injuries, 3 or 4 times. There is one boy who is a nasty verbal bully and wind up artist - even I have come under fire from him. From what my son tells me, verbal and physical defiance and bullying has stepped up a gear this year - and this is at a small church school with very strict codes of behaviour. I don't know if this is to do with the age of the boys and the pressure of being in year 6.
I too can't face the parents. One of them gives my sons a lift home in her car (a paid arrangement) and I just can't bring myself to talk to her now.
My son is lucky in that for him there are more friends than bullies in his class. What about your son's friends at school? do you know the parents well? can you talk to them and between you, get your son's friends to stay with him more - that's if you consider letting him go back for the last week or two.
I am saying this because of all the end of term events that will happen. I really feel for you and your son, missing things like the leavers evening, class parties, year 6 play or whatever goodbye events your school puts on.
If you decide going back is just not worth it, it's totally understandable. I am just so sorry you'll miss the nice bits.
Mind you, the bullies at our school will not necessarily have nice memories of their last day at primary. My son tells me, lots of children in his class, including him, are determined to fight them back on the last day. The reasoning is that they will have nothing to lose, as they will be leaving and the bullies are not going to the same secondary schools as they are. I wonder if it's common for year 6s to threaten anarchy like this?
Anyway, I am rambling - I will watch out for your messages and I really hope you get a speedy answer about de-registering and your son feels tons more happier very soon.