For those of you who don't know, this is the exact wording of the First Amendment of the Constitution of the United States of America (meaning it only applies to America)
Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof; or abridging the freedom of speech, or of the press; or the right of the people peaceably to assemble, and to petition the Government for a redress of grievances.
It means that the U.S. government cannot pass laws against people speaking, nor can they arrest people for saying things they(the government) don't like. However, that doesn't mean you can say whatever you want, where ever you want without any consequences. There are laws against libel and slander and verbal threats against one's life.
Those kids who called their teacher a pedophile and a rapist were guilty of libel. Plus they could have endangered that teacher's career, reputation and even life with their (the students) posts.
They got suspended/expelled, but they haven't been punished by the U.S. government.
Those kids, and many other people, need to learn that what they say and write has consequences