Tethersend's advice above is good, but I wanted to reply on this:
"The new headmistress TOLD me I had to remove this or she souls report to police for slander."
Too often schools will hide behind this kind of threat. They don't want parents spreading bad news about anything that happens in the school. If what you've said is accurate and fair, and you know that it's true, then feel free to stick to it. Other parents have a right to know what goes on at the school and facebook is a good way of telling each other.
As for Monday's meeting, keep your cool and let them do the talking to start with. If you have a partner or friend you can rely on, take them with you. Keep a notepad in front of you and note down points made. Know beforehand the questions you want answered, and don't allow them to give evasive answers.
Teachers are cool and professional, so however cross you get, keep the tone formal if you want them to take you seriously.
There's tons of advice on the internet about how to do meetings well, you could take a look at that too. Try to be dispassionate and not get hot under the collar because it's your child.
Best of luck, and let us know how you get on.