J.N.
A couple of things to keep in mind (from a former elementary school teacher): The first of the school year is very hectic and busy for teachers. And a teacher's day NEVER ends when the kids go home. Especially right now, it's easy to get caught up with all the preparations that have to be made (some of them before the kids come, some can't be done until you actually meet the kids, etc) and forget the time. And, if she's like me, she's probably feeling stressed with trying to balance being a mom and a teacher, and get everything done on both sides. And having a mixed up babysitter situation (switching babysitters, plus having this gap in between, where you are filling in) just adds to the stress! (I've been there, too )
Not that it is easy for you! And you do have the right to tell her you need a much more definite time - if she tells you 5, she needs to be there by 5 or call you. Just let her know that you need to be able to plan on some specifics, so that you can be ready. And it's hard for a 2 year old to not know when mommy is coming.
I just wanted to give you the other side of the coin to look at.