According to the article there was a specific script to be followed when distributing the survey. By informing the students of their 5th amendment rights he deviated from this script. Beyond this it was obviously the intent of the school system for students to complete the survey fully and honestly. He knowingly took actions which he knew might lead students to not doing so.
The article didn't say that he did not read the script - just that he also offered information about the 5th amendment. That's not an instance of failing to fulfill his obligations. Also, even if he acted in a way to thwart the "intent" of the school system administration, which has not been demonstrated, it is not at all clear that this constitutes a failure to fulfill his obligations as an employee. It might be that his obligations should be interpreted as including the thwarting of the intent of the administration in some instances. Just as a soldier in the American army is obligated to disobey an illegal order.
