New York City's standalone middle schools do a worse job educating students than schools that offer kindergarten through eighth grade under one roof, according to a new study to be released Wednesday by researchers at Columbia University.

On average, children who move up to middle school from a traditional city elementary school, which typically goes up to fifth grade, score about seven percentiles lower on standardized math tests in eighth grade than those who attend a K-8 school, says Jonah Rockoff, an associate professor...