For the average level of body awareness in the classes I teach, the instruction to relax all muscles other than the antagonist of the muscle under stretch means very little.
I have found that the instruction to stop the action that holds you out of the stretch means much more and is much more actionable.
Even then there are some class participants who insist on having something to *do* (as opposed to *stop* doing).
I have found humour and other positive distractions (like discussing the anatomy of the body part being stretched) are a very good way to circumvent the 'apprehension reflex' as are the use of supports (bolsters, blocks) to provide feedback.
In partner stretching the apprehension reflex is reduced significantly if the assisting partner does not fidget and provides a persistent and insistent weight or force (good partnering is a crucial part of Stretch Therapy in my humble opinion).
When all is said and done as a teacher the best you can do is provide a safe environment for effective stretching.