The key word in this story is that it was a joke. Furthermore, the phrase in the story also says that students put the duct tape on their own mouths. But the important thing to remember is that the students and the teacher were in agreement that this was a joke.
With all the junk and ugly things that so many teachers do to our students and vice a versa. It seems absolutely ridiculous for a school board not to understand the difference between a joke and something that is serious.
Obviously if the children would the duct tape on their own mouths , then obviously there right of privacy was not in fact impaired or compromised.