"...he has heard her lecture, for 20 minutes, that she and the rest of the staff don't want him there"
Then he would need to hear her say, "I apologise, I had absolutely no power to make such a statement" and frankly, I don't see that happening. If the director had any guts, she would have stepped in long ago and made this teacher eat her words. But for tis teacher to say this, so emphatically, and repeatedly for so long? She feels mighty confident that what she says, goes. And why does she feel so confident? Because somehow, in some way, she knows the director will not stand up to her.
I agree that he needs closure - what about you going in with him to say goodbye? He doesn't stay for a full day, he goes in with you (prepared ahead of time by the director, to make sure there will be no problem from the difficult teacher) to clean out his desk, to maybe stay for long enough to play at recess, then leave on a good note? That has to be much healthier than the current situation. And if that creepy teacher insists he has to leave on the bad note she threw him out on, so he will "learn his lesson", then i do think the director has to stand up for difficult child to insist on a good closure. It's bad for the school to see what they saw the other day; it's bad for the school to have parents able to say. "that school threw my son out, he left on a really bad note."
I really think this director sounds very gutless, when it comes to dealing with this teacher. There can be many good reasons for this, including politics of who actually runs the school,' who funds it, who is connected to whom in the town - when you really need someone, you will put up with a great deal from them.
We have a new principal coming in at the local school here. The previous principal was Acting principal, for 18 months, and did a really good job. However, because he was Acting, he was restricted. The previous (not Acting, but permanent long-term) principal was a lovely bloke, a good friend, but a total wimp when it came to standing up to his staff. I had to pull my kids out of there - three times. The fourth child, they refused to enrol because they saw her as a liability, so I sent her to another school that saw her as an opportunity. I could have legally forced the issue and made them take her, but they would have one way or another got rid of her, or "found the evidence" that it wasn't working. The situation was too delicate so we moved her.
On the subject of applying leverage to your director, if she continues to say that difficult child doesn't need a 1:1, then point out that she may well have been correct (although why did she have a 1:1 in her own school?) but now he has been traumatised and damage done, it is going to take careful supervision and a lot of 1:1 patience to undo the damage she allowed to happen in her school, by NOT keeping that teacher away from him (or sacking her) as should have happened. You gave her fair warning that there were problems; the current situation was entirely forseeable, which makes it legally actionable should you choose to sue.
Of course, you don't want to sue. All you want is a 1:1 in the public school, where you shouldn't have to send him, but for the mess the director has allowed you all to get into.
Good luck, let us know how you get on.
Marg