I agree with the previous poster. My family time is precious to me, and as a working mother I feel I owe it to myself and my family to leave work at work whenever possible. Sure, we all continue to think about school issues at home, and many people bring work home to do (I choose not to), but there needs to be a boundary that allows you to refuel and rejuvenate yourself so you are able to be your best for yourself, your family, AND your job. If you are uncomfortable, then that is what matters. It needs to stop. I'd personally do the letter as mentioned above and also the answering machine. I use my machine as a screener as well and pretty much don't answer to phone after 5 PM. If it's that important they will leave a message and I can get back to them at a time that isn't interrupting my family time. I'm all for having fun at work and visiting with coworkers throughout the day, but it sounds like this woman is monopolizing your planning time as well. Maybe take the more agressive approach with that piece of this situation. If you close your door in the morning does she still come in? Sheesh! Good luck to you.
PS I didn't think you sounded defensive whatsoever.