IN 1927, a schoolteacher in Secaucus, N.J., named Helen Clark lost her teaching license. The reason? Somebody had seen her smoking cigarettes after school hours. In communities across the United States, that was a ground for dismissal. So was card-playing, dancing and failure to attend church. Even after Prohibition ended, teachers could be dismissed for drinking or frequenting a place where liquor was served.

Today, teachers can be suspended, and even fired, for what they write on Facebook.

Just ask Christine Rubino, the New York City math teacher who may soon be dismissed for posting angry messages about her students. Last June, just before summer vacation began, a Harlem schoolgirl drowned during a field trip to a beach. Ms. Rubino had nothing to do with that incident, but the following afternoon, she typed a quick note on Facebook about a particularly rowdy group of Brooklyn fifth graders in her charge.

“After today, I’m thinking the beach is a good trip for my class,” she wrote. “I hate their guts.”

One of Ms. Rubino’s Facebook friends then asked, “Wouldn’t you throw a life jacket to little Kwami?”

“No, I wouldn’t for a million dollars,” Ms. Rubino replied. She was pulled from the classroom in February and faced termination hearings; the case is now with an arbitrator.