Alex Osborn, known for founding the Creative Problem-Solving Institute, set up a structure for group creative brainstorming. One rule to free people from creative inhibitions forbid criticism and judgment during initial brainstorming.  No need to guess why. People tend to champion their own ideas at the expense of the better ideas. It only takes a small minority set on “defending turf” to have a detrimental effect.
Ask educators the way to encourage creativity and they will tell you students need an environment full of stimuli that highlights innovation in the arts and sciences. They will also say the classroom should have an open, accepting atmosphere, free from criticism to build up self esteem. Ask people noted for work in creative fields, and they might agree with a stimulating environment, but not the lack of criticism.
When I interviewed college art majors and educations majors to describe factors lead to creativity, both groups placed willingness to take risks was high on their lists. The art majors said being around creative people was the most important factor. The teachers gave high self esteem first place; however, self-esteem wasn’t even mentioned by the art majors. They preferred honest critiques of their work. Evidently methods used to build self-esteem, do not necessarily build creative thinking.
Teachers are have difficulty with highly creative students, for same reason corporate workers find creative colleagues difficult; they tend to level criticism at others more frequently . But evidently they are okay with receiving criticism, too. Both innovative students and employees seem to take the anxiety resulting from a negative evaluation of their work and convert it into a drive to be even more unique. 
Research conducted in both the United States and France, examined the result of brainstorming groups when one was told not to criticize while the other was encouraged to debate ideas. Those allowed to debate generally came up with superior ideas. The key was to encourage debate, and even criticism, without allowing anyone to monopolize the session.
“Researchers of group creativity have noted problems such as social loafing, production blocking, and especially, evaluation apprehension. Thus, brainstorming techniques have specifically admonished people ‘not to criticize’ their own and others’ ideas, a tenet that has gone unexamined. In contrast, there is research showing that dissent, debate and competing views have positive value, stimulating divergent and creative thought.”