Yes, Romney did make that claim in his educational policy speech. Here is the passage:
We have good teachers, like the ones who are leading New York City’s Democracy Prep. Because of them, kids from the city’s poorest community are outperforming children from the wealthiest. Last summer, these teachers took over the worst elementary school in Harlem rather than let it shut down. Democracy Prep is a testament to good people who refuse to give up on our kids or leave our cities without a fight.
That worst school in Harlem, taken over by Democracy Prep? None other than Harlem Day Charter School.