Good questions.
We had a talk and the uncertainty of the future juxtaposed against the present school/SS no book store job started to look a lot less viable. She now thinks that working to FRA is better and then look into SS at FRA. If she wants to earn more than that "working and collecting SS" level then it won't matter. She also is going to investigate the work center, can't remember the name, that someone suggested. As bad as the book store situation is, quitting and collecting SS has potential issues that can't be ignored.
If her heath is good, she most probably will work well beyond 70 at the school because she really enjoys it. One of her friends at that same school retired by choice at the end of the school year this past Spring at age 78 so it doesn't appear they force you to retire based upon age but then that person was in the union and my sister is not.
It all looked good on paper but when figuring in potential problems, this idea does have downsides. Thanks for the feedback everyone.