Sorry to hear that.
As a start, sow about looking at your facebook account and making copies of the page(s) and posts that you think are the ones that are causing concern? You need to have a record of what they actually said so that it can't be turned into something much worse by chinese whispers or bad memories etc. if you use firefox there is a great extension/appn (sorry can't remember what they are called) called ScreenGrab! that adds an option onto your right mouse menu and it will get great screen grabs of entire pages for you very easily.
I'd then take everything off for the time being - not sure if there is a way to go inactive for a little while so that you can go back on it later but so that you can be shown to be pro-actively reducing anything that they thought might be controversial. At least set the privacy settings to the highest you can as a minimum
How many people are you friends with on fb - some good friends or lots and lots of people that you know vaguely but that will know that you work in a school (and therefore even if you do not mention it they will know what you are talking about?)? Unfortunately it is different from venting on something like MN where you are anonymous (assuming your nickname is suitably vague), on fb it's all about the person.
do you belong to a union you can talk to or have legal insurance on your house or car insurance that you can to a lawyer through and get advice from, otherwise how about CAB?
Do you know who might have reported you? Or how they know about your comments? Were you expecting anyone from school to see them / comment on them? Do you know if anyone else made any comments on facebook about the event - and if so, get screen grabs of them as well. Are they concerned with the content of what you put or the fact that you put anything at all? its always worth writing down now why you wrote what you wrote, who you were expecting would see it, who would be interested in it and why, who would not be interested in it and why, your feelings at the time, your feelings now, anything else that can possibly be related to it... Send it to yourself in an email so that it is effectively dated and keep a print out of it (along with print outs of the screengrabs) and keep a file with everything in separately to keep yourself organised with this.
Did something happen at the event that you can use to counter attack them with - that eg it was badly managed and went badly which made you look bad but you felt that if you said anything at the time you would have been picked on/had problems/been bullied etc. Therefore if they had been reasonable and you had been able to tell them xyz you wouldn't have needed to vent on FB etc -
It might also be worth googling to see if you can see other cases where facebook has been used to sack someone and see what reasons/bits of law were given. Then think how that applies to your case and what your response would be if the same was applied to you.
Finally (sorry, seem to have gone on a bit) re-read your terms and conditions and any work policies to see what policies they have about using facebook or mentioning the school in public or whatever so you know how they specifically are going to be 'getting' you and if you can argue that you didn't know any better as there was nothing in your t&c to suggest that this would be an issue - or that they are being over subjective in their interpretation of the t&c, whatever it turns out to be.
good luck!