I believe that the current education system constrains the imagination in large part because of its focus on specific degree programs. If they were centers of learning, they would encourage students to pursue many different areas they wouldn't otherwise pursue. However, colleges force students to complete a certain set of course work in a certain degree path, which constrains their imagination.
The current education system in America is governed by two principles that keep creativity low - efficiency and simplicity. Complex forms of education and ways of teaching people different ways of thinking are ignored in favor of a strict curriculum designed to keep schools funded by way of standardized tests.
The educational system in the United States does not reward creativity. Thanks to No Child Left Behind, schools must teach tests to children. They teach them uniform facts, statistics, rote memorization. There is a system to follow, and deviation from that system is not tolerated. Whatever creativity a child has left after high school, hopefully they can express in college.
The current school system teaches kids using predetermined assignments and tests. These assignments and tests contain questions that the students are expected to all give the same answer to. If you try to give a different answer than the one called for by the teacher, you receive a poor grade. This teaches kids to conform, and to answer as expected. This kind of education has very little room for creativity.
I think the current modality of learning and teaching, for the most part, constrains the imagination. For some students, it is the perfect structures. But the thing is, we are all wired differently. For example, there are many brilliant dyslexics with great imaginations, but under the current modality of learning, they faulter.