Recently, a high school clarinetist sent me a letter, in which he asked me a few questions. One of the questions was about how to play high notes. This is a difficult question to answer in a letter, especially when you aren't able to see and hear the person playing them, but here is my answer. And while it is fairly general, I thought I would include it here:
"...when you are working with very high notes your embouchure will need to alter slightly. The lower lip must roll out slightly to allow the reed to vibrate intensely. This will have the effect of flattening your chin muscles firmly against your teeth. Your jaw must project out towards the ligature: form the vowel “Ee” inside your mouth with the tongue higher towards the roof of your mouth. Hold the clarinet closer to your body so the wedge shape of the mouthpiece applies more pressure to the reed while allowing more vibrating surface. Push up with your right thumb to involve the mouthpiece and reed deeply into the sound. This will help tone quality. Practice chromatically in the altissimo and learn the alternate fingerings which will aid facility and help intonation.|"