It's not like their life depends on this... oh wait, it does.
Central to my teaching is the concept of "limitations". Before we even get into the pool, we go over my three rules of Scuba, the second one being the "Rule of Fun": You can call a dive at any time, for any reason, no questions asked and no repercussions. IOW, if you're not having fun, then we're doing something wrong. Let's stop and figure it out. We go over this with great care, covering limits such as time, depth and air as well as conditions, stamina, experience and training. The whole point of training is to teach them what is appropriate and what isn't. I actually endeavor to impart judgement to my students. That doesn't mean I don't hold my students' trust of me sacrosanct. I know not every instructor is as conservative as I am. They aren't as good looking either!

So, I make sure that my students feel comfortable raising concerns and usually I'll throw them a dive plan that is outrageous and see how they react. If they aren't willing to call a bad dive plan, I'm not willing to give them a c-card.