I agree, husband should be in there with you. I know it's sexist, but when a mother complains they're more likely to be dismissive of a "highly strung mother" and less likely to be dismissive of an "authoritative father". Life isn't fair.
I would be in there slugging away for difficult child. For years I bent over backwards to make it clear to difficult child that although I knew he was being bullied, if he had not been doing something wrong (like hitting back) then he wouldn't have been the one in trouble; finally, years later, I realised I had given the school too much 'wiggle room' and in the process, been extremely unfair to my own son. The school had completely had me bamboozled, even when I thought I was fighting for my son successfully.
In this, I would be believing difficult child. And if the school comes out with, "Well, did you know that your difficult child didn't tell you the whole story? Did you know he was really doing this, that, etc..." then DO NOT immediately get outraged at your 'deceptive son' - this is a common tactic for people who want to deflect an angry person from attacking them - they redirect your anger back towards your own home front in an attempt to defuse your attack on them.
I'm not saying to totally disbelieve them on principle; just take notes, talk to difficult child later on in a way which does not prejudice his answer, but in the meantime while at the school (and do not do this over the phone) insist that they stay on topic, and try to compromise with, "I will discuss this with my son and hear what he has to say about this. In the meantime, would you accept that IF what my son is reporting is correct, this would be a very unfair situation requiring investigation and a change in strategy?"
Follow up any such meeting with a letter (again), minuting the meeting. Simply say, "Thank you for seeing me on [give date] about [describe situation]. At this meeting, I was concerned about [state issue] and you told me [what school said]. Our outcome was [give what you think outcome was]. If I have somehow misunderstood, please correct me, in writing, by [give date, no more than a week away]. Since our meeting I have talked with difficult child and he told me [describe whether difficult child confirmed the school's version or insisted they were wrong and he was right]. I do believe that this is how he perceives the situation and I am further concerned that this is leading to school phobia. It is in both our interests to resolve this to mutual satisfaction, in order that difficult child may feel safe and supported in the school environment, and in order for him to be more likely to behave and cooperate in class. I feel we should meet again to discuss how this can be accomplished."
If the meeting outcome is a stalemate, still follow it up in writing and include your concern that you cannot reach agreement with the school on what has been going on, and what they are going to do to resolve an unsatisfactory situation. Always ask them to confirm or refute in writing and indicate that if you do not hear from them in writing, you will take that as confirmation that your record of the conversation is correct.
Then if you still get no satisfaction, you have a written record of events to date which can be used to hopefully get action from higher up.
What SHOULD happen is that they will take you seriously and maybe make a decent effort to not have difficult child feel so victimised. Even if it's all in difficult child's imagination, if they can help him feel more supported (and hey, it doesn't take THAT much) then it's got to help.
If difficult child is right, and they have been making him a scapegoat, then having it all in writing is enough (hopefully) to make them realise that you are not going to be easily fobbed off with platitudes and misdirection, and that even if difficult child is a horrible little *****, they need to treat him with kid gloves. This isn't fair, but it IS what happens. But if the school is right, and they give way to him just to keep you happy, eventually you will see enough in his behaviour at home to realise that he's getting away with murder at school, and you will be able to have another talk with the school to set something else in place, more supportive of the school.
But frankly from my own experience and what you describe (plus a lot of what you've written about him in the past) I do not think this is him trying to make the school look bad just to cover up his own misdeeds. I think this is a kid genuinely believing he's being victimised. And he needs you to at least show that you will take his side. He needs to know he can come to you when he's in trouble, and you will help him. He also needs to know that bullies can be faced down and there are good and right ways to 'win'.
It requires not only the first meeting, but for you to stay on their case and keep monitoring the situation not only by talking to difficult child, but if you can, find another kid in the class who is neither part of the chatterbox group nor bosom buddies with difficult child, and who is prepared to talk to you confidentially about what is really going on. I found I got my best information from 'spies' in the classroom - kids mostly, and occasionally adult volunteers. Bad teachers will often be slack in other ways, letting their guard down in front of kids. But if you do use another kid this way, do your utmost to keep their name out of things unless they choose to report a problem themselves. Once a 'spy' is outed, they lose their usefulness because teachers will know to guard their tongues around such a kid. They can also be victimised (again, by bad teachers). This happened once with us - the son of my best friend was victimised in this way.
I'm sorry this sounds so cloak and dagger, but it is what we had to resort to. And not just us - I hear this over and over, different schools and different teachers. As I say, good teachers have nothing to fear.
The biggest use of kid spies, I found - to get an overall feel for what is REALLY going on, so I could better protect my child, or at least interpret what REALLY was happening instead of what I was being told. Getting a better picture, a more detailed and accurate picture, helped us avoid the kids who were the biggest problems and made it easier for us to jump ship when the time came. A more educated, informed decision. And a big hug to my former spies - those who I'm still in touch with have all grown into smart kids who are doing well in school and who still look out for difficult child 3 when they see him. They were justice-minded back then and it is showing now, as they mature. And still only they know how much they helped me understand the true picture.
Marg