The twins. (School District 146 handout photo)

The eighth-grade graduating class of a middle school in suburban Chicago had nine sets of twins — that’s 18 out of 268 students, or a little over 7 percent — but officials didn’t realize it until a parent figured it out last month.

The Chicago Tribune reported in this story that three sets of the twins at Central Middle School were identical and six were fraternal.  Principal Randy Fortin was quoted as saying that none of the twins got into any real trouble, although earlier this year, one boy took a math test for his brother.

 “At the time it wasn’t very funny, but now it [is] funny to talk about,” Fortin said.

Shortly before the eighth-grade graduation, a parent went to school officials and told them about the nine sets of twins, but Fortin said he didn’t believe it until school records confirmed it.

According to this 2012 report by the Centers for Disease Control, the rate of twin births jumped 78 percent from 1980 to 2009; in 2009, one in every 30 babies born in the United States was a twin, compared with one in every 53 babies in 1980. Even with that rise, however, the Central twin population is unusual.

One question: How did nobody in the school realize it until graduation?