My mom is a high school teacher in Missouri, and this law has far reaching effects, not just with regards to Facebook.
For one, the school district provides students and teachers email addresses in order to facilitate school related communication between the two in regards to homework, etc. My mom would have students email their papers and assignments regularly. Not anymore, according to this law.
What about school clubs wishing to create a Facebook page or other online presence? If it's a school-sponsored club, the school can be held liable for anything put online, yet the faculty sponsor would be putting him/herself at risk by accessing the page to ensure that everything's ok.
Our legislators need to stop trying to legislate for every little thing, and start ensuring that our government has the resources it needs to enforce the laws we already have.