You chose to miss 3 days of school
You did not ask
Your parents did not request this
You, presumably, did not go to the teachers before hand and ask for catch up material
You just decided that you knew better than the school
That was your choice
But now it seems that the Head of Maths has a choice whether or not to accept someone with this attitude onto the FM class
He has not said "no", he has said that you can attend the class whist continuing with your other course, whilst you take the chance to show that you have an attitude he is happy with
Of course, your parents could threaten to take you out but that is not really going to happen at this point in Y12, is it?