Is it possible to convince a group of people that they master something, when they in fact don't?
Your question relates a bit to the self-efficacy literature.

Underlying ability is one cause of self-efficacy. And presumably, there are many mechanisms by which people can form an accurate opinion of their ability.

That said, there are various known ways of increasing or decreasing self-efficacy without changing actual efficacy.

For example, there is the big-fish-little-pond effect. Basically, you can increase self-efficacy by putting people in an environment with others who are less capable. 

Similarly, giving people feedback that they are good or giving people tasks suited to their ability will increase a sense of self-efficacy.
Some of these strategies are considered effective teaching especially if it helps overcome obstacles related to students even trying. But it can also be problematic if people allocate resources based on an inaccurate belief about their strengths and weaknesses.