Math can be hard enough, but imagine the difficulty when a teacher is just one chapter ahead of the students. It happens, and it happens more often to poor and minority students. Those children are about twice as likely to have math teachers who don't know their subject, according to a report by the Education Trust, a children's advocacy group.

Studies show the connection between teachers' knowledge and student achievement is particularly strong in math.

"Individual teachers matter a tremendous amount in how much students learn," said Ross Wiener, who oversees policy issues at the organization.

The report looked at teachers with neither an academic major nor certification in the subjects they teach.

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