Will I be excluded?!?

[Hannah]

I made a comment on my (private) twitter account about the fact that my head teacher is a "bitch" for making us come in on a snow day, and my parents received a phone call saying that when we return on Tuesday I'll be called into my head of houses office and "questionned" about it, and talk of being potentially suspended. I was just wondering if this was allowed? My twitter account is private and the school does not follow me so I am confused as to how they'd have seen it, but nether the less they have. Can I be reprimanded for this? If was an indirect comment (she was not tagged in it) and also was not intended/permitted for the schools viewing. It was also a joke, and I feel it is an infringement of my privacy, as I didn't wish for them to read it. Also, does my freedom of speech apply in this situation? Someone shed some insight onto grounds of suspension please? I am a very good student with no history of any trouble and I want to know if this will cause me big problems or do I have a point in that they shouldn't have been snooping on my private life/tweets?
Thankyou! I am from the uk if that helps
By private account, I mean that I changed the privacy settings so that only selected people can access and read my tweets, if my account wasn't on private I'd never have done it.