I think the OP's self imposed rules are pretty reasonable, if she is willing to keep her own profile and posting squeaky clean and fairly impersonal.
My general Facebook rule is that I only post things that I am comfortable with everyone in my friend list, and I only friend people who I am comfortable seeing what I post. For me, that means I only friend people who are personally known to me, and I generally don't friend current coworkers.
Not friending anyone not related to them who is still in high school and/or a minor is also a good blanket rule for teachers, particularly if they aren't willing to carefully police everything on their site, and be wizards at the privacy settings.
These rules are partially there to protect the students by making sure that the teacher doesn't cross inappropriate boundaries. But mostly they are there to protect the teacher, as the teacher can be held accountable for what they present to their students. A teacher also has a higher responsibility to report the behaviour they witness. Although personally, I think anyone who friends their teacher and then posts pictures of illegal activity (underage drinking, for example) is asking for it.
I find that by graduate school the boundaries have changed completely - I would go out drinking with someone who taught my course or was my supervisor without a second thought.