There is an advanced course for experienced lip-readers who want to become intructors. If they qualify, they will teach lip-reading classes themselves.
All the participants in this course-for-instructors have excellent sight, hearing and other senses. They are all high-level lip-readers. They are all currently in good health and in top form.
Today a group (lets say 26 - it's not important) of them are sitting in a circle facing inwards. It's a clear sunny day and everyone is in view of everyone else.
Their instructor explains the process and they start.
Person A turns to person B and, hiding their face from all but B, silently mouths a single word, for example "hippopotamus", or any other word. In the same fashion, B mouths what they guess the word is to C and so on, all around the circle.
Finally person Z mouths their guess to A then asks out loud, "Was that your word?"
Person A truthfully replies out loud, "I haven't the slightest idea!"
Everyone laughs then they turn to the instructor for feedback.
The instructor says "Any questions? No? Excellent, you all pass that exercise! Let's move on.
Please explain the purpose of the exercise and give a convincing reason why they passed. Optionally can you say something that is likely true about the location where this class took place?
They are all speaking the same language and they are all fluent native speakers of that language. Arbitrarily I'll say that the language is British English because that is where I'm from, but it could some other normal, spoken language.
Everyone has an excellent memory.
No-one is drunk or otherwise incapacitated.
(added) You can assume that the original word is easy to lip-read correctly for these people.