So let's count the number of adults who have seen fit to engage with this piece of teenage fun, and we can also try to estimate the amount of time the 'adults' have wasted on it. I don't count the OP among the 'adults' in quotation marks, only the teachers and school admin.
The OP finds herself caught in the middle of an incredibly silly battle of wills, fought by incredibly silly people who are all equally up themselves. Because of her age and immaturity the teenager is excused from responsibility here. The teachers and school administrators have no excuse whatsoever for their gobsmacking performance.
Here we go:
I got a phone call about half an hour after school starts to say she had entered school with it flipped up and has gone back to normal lessons.
1 (whoever phoned)
Her 3rd class teacher reported her.
+1
So she was sent down to the office for isolation and I got another phone call.
+1 (office staff)
I had to go in for a meeting after lunch and miss work.
+1 (and you were obv on the other end of the morning phone call)
Basically the headteacher and her head of year have said, she is not as of yet suspended, they really don't want to suspend her (don't think it looks great on their records) and as they think progress was made this morning when she flipped it up, they are giving her until Monday,
+2
..if she shows up on Monday with it in, I have to pick her up immediately, and she gets a fixed term suspension.
6 adults, assuming no duplication of effort.
I would estimate at least an hour of attention overall form the Year Head and HT.
10 minutes from whatever teacher took note and phoned in the morning.
10 minutes from the 3rd period teacher.
? from the office staff.
Maybe four hours of missed work by the OP so far (?) and more threatened, because the school thinks its ridiculous rules are more important than someone else's livelihood...
The school has completely lost sight of its function.
This is preposterous, and the comment that the HT and Head of Year are weighing how this looks on the school records speaks volumes about what this kerfuffle is really about.