Cover your back - go the GP and say that you would like an assessment with someone over your child's recent behavior. Keep a home bahavior diary. Do NOT share this with school. Include your child's "mood" before and after school.
Ask school to keep a simple behavior diary for you listing any incidents of amazingly GOOD behavior and bad behavior. Ask them to state what the environment/activity was like immediately prior to the behavior concerned. Tell them this so that you can show it to the specialist when your GP's referral comes through.
IF a SN is causing this behavior it's will help the professionals sort it out. If it's simply a poor TA or teacher/bullying by other kids etc causing the poor behavior this will also enable you to start ferreting it out. Make damn sure your child knows you have his back and love him, even when you are disciplining him. The closer your relationship with your child, the more you'll be able to ascertain what's REALLY going on, even if your kid is non-verbal. Your child needs to have ABSOLUTE trust in you.
Do NOT tell the school you think they are lying - even if you know damn well they are. Look for evidence but keep your powder dry.
Your diary will throw up some interesting patterns over time that are sure to suprise you. e.g school may not tell you these incidents only happen when your child is asked to work in a group with kids d, e & f for PE. Once you know this a little digging on your part may throw up that this is when these kids can bully yours without being spotted by the adults and that is what is causing the explosions of your child.
The home school behavior diary is a reasonable request to make of the school in the circumstances. You can offer to add a note in it on days when your child doesn't get much sleep the night before, or is feeling under the weather. That's how I'd approach the meeting.
Apart from this request say as little as possible.
Having been the victim of a witch hunt of this nature myself, my next step would be to source alternative educational provision for my child. I wouldn't discuss it with my child's current school until it was secured though if at all possible. How you go about this depends on your child's SN, available provision, your relationship with the LA etc, etc.