Place your left hand on their head, and with your right hand push the clarinet firmly and gently up against their Ptop teeth. They will then feel what you mean when you tell them to have the top teeth touch the mouthpiece, and will be able to continue to follow your instructions. Have the students start reading the notes they are learning by the second lesson. Do not delay this. This is important to their sense of accomplishment, and you want them to recognize fingers, notes, and sounds to the notes on the page.
Good posture is important.
If a student is having a hard time keeping up with the notes in the book, help them out by pointing the notes out to them as the class is playing. From the time the students can produce a stable sound or even just begin to do so, tune them. Make sure the are taught to listen to themselves and others immediately. You cannot tune students the night of the concert and expect the students to play in tune during their performance. If they have not learned to listen everyday, they will not be able to do it on a concert when they are nervous.