It would be in most unions contracts. Falls under a similar line of using the school computer for any illegal activity.
While Peter's response sounds tongue and cheek, the fine line between what is appropriate and isnt for a teacher is a good point.
I am not allowed to smoke pipe at school (obviously), but if I am around town smoking and one of my students see me, is that grounds for discipline? I don't think so (and I think the district would have a real hard time doing anything like that). However, if I am busted on my own time for meth use or something like that, it is a different story.
Item four of the NEA (National Education Association, one of the two main national organizations for teachers_ Code of Professional Ethics states:
[The teacher...] Shall make reasonable effort to protect the student from conditions harmful to learning or to health and safety.
Being a methamphetamine addict would likely pose a risk to health and safety. Engaging in prostitution could be argued to as well.
In particular, I work at a day treatment with students with severe behavior disorders. Many of the students have also been sexually abused or perpetrated sexual offenses themselves. Having a prostitute teaching them would probably not be the best idea.
Anyway, interesting story. I am particularly curious if this was her first time. The new article says her record was clean till now.