Teacher crush creativity because the qualities that make a person creative also make them hard to handle in a classroom.
One of the most consistent findings in educational studies of creativity has been that teachers dislike personality traits associated with creativity. Research has indicated that teachers prefer traits that seem to run counter to creativity, such as conformity and unquestioning acceptance of authority (e.g., Bachtold, 1974; Cropley, 1992; Dettmer, 1981; Getzels & Jackson, 1962; Torrance, 1963). The reason for teachers' preferences is quite clear creative people tend to have traits that some have referred to as obnoxious (Torrance, 1963). Torrance (1963) described creative people as not having the time to be courteous, as refusing to take no for an answer, and as being negativistic and critical of others. Other characteristics, although not deserving the label obnoxious, nonetheless may not be those most highly valued in the classroom.
From Creativity: Asset or Burden in the Classroom?, a good review paper. What the paper shows is that the characteristics that teachers use to describe their favorite student correlate negatively with the characteristics associated with creativity. In addition, although teachers say that they like creative students, teachers also say creative students are "sincere, responsible, good-natured and reliable." In other words, the teachers don't know what creative students are actually like. (FYI, the research design would have been stronger if the researchers had actually tested the students for creativity.) As a result, schooling has a negative effect on creativity.
Like Alex Tabarrok, I don't really blame teachers... it's hard to control a class full of children. The thing I take away as a parent is that I need to make sure not to enforce the rules so strictly that I stunt my daughter's growth even when she's not at school.