Looking up at a contrail is not the same as a direct violation of clear standards prohibiting shutting off a student's air. That could easily result in someone being seriously injured or worse. I am known for teaching above standards within the standards of the agencies I certify divers through. One agency actually requires me to add material and allows me to test on that as a condition of certification in our local conditions. There are some things however that fall within the realm of unnecessarily dangerous. Those things don't get done in open water. Air depletion in the pool is risky IMO. You have to be in direct contact and ready to respond instantly. In open water it's just nuts to do.