Open Question: How can teachers see students facebooks?

At my school, they recently banned PDA's and no one was happy about it. Someone created an event of a whole school group hug and some people from the school attended. The teachers somehow found out about it and come to every class in the school and went off their faces to the class about it. They have a list of all the names and comments that were posted. How can they do this? How do they get onto our facebooks without it being pedofile?