Nechomah- They absolutely do have the right to make the rule, but not to reinforce it with expulsion.
But look at it from the students perspective: The school was willing to have all the girls make Facebook accounts- and they most certainly knew the potential dangers of it- to make some money. And then they say that anyone who does not delete their account will be expelled.
They are showing the girls that they are allowed to compromise their values to make a few bucks. It is simply unfair and hypocritical.