There are some instructors who will remove regulators from students' mouths during instruction, but the practiced is actively discouraged by all agencies because of the very real danger that it will lead to an involuntary swallowing of water which can trigger a gag reflex which can lead to all sorts of problems, possibly including drowning. In a controlled environment, any diver should be able to find a lost regulator, use an alternate, etc. In an uncontrolled environment, including possible fear of further attack, it is not so certain.