Based on every crash that I have witnessed out on the range, none have been a high side due to releasing the rear brake. Every crash has resulted from using too much front brake resulting in a front tire skid, and as a new rider, natural reaction is for them to keep them locked even though we tell them that you must immediately release the front brake once your front tire begins to skid. We have rear tires lock up all the time during the quick stop excercise, and none have resulted in a crash, just because they keep it locked. We just had a rider crash this afternoon during their skills test because he locked up the front brakes.
We are talking about new riders here. Most of you have way too much riding experience thats making it difficult for you to understand that in order for them to learn how to ride, they must acquire the basic fundamentals of braking and learn how to stop in the shortest amount of distance, which is using both brakes. As they progress and become experienced riders, they will naturally progress to adapting other ways to stop quickly based on various conditions and capabities of the bike and rider.
By showing them or telling them to use only one brake when they are learning how to ride is like telling them to only take left turns.