I think Rules are rules....and there should not have been an extension.....but my guess is that the principal will state that it is up to each teacher. So, I doubt that you will win this one......
Let's just hope the other girl loses and it teaches her that there are rules to live by and you don't always get what you want.....
I would talk to the teacher and ask her why she made this exception, if this is true......
Good Luck and take care. Also, use this as a lesson for you child.....sometimes people do things that are not correct, thus hurting the other people that they may not think is affected....so when you set a deadline, hold to it.....and you can also teach her that some people take care of the people they like....not saying the teacher prefers the other child, but that she did this for a reason.........