Florida’s commissioner of education, a rising star in a national movement pushing for test-based accountability in public schools, resigned on Thursday after just seven months in office, after news reports surfaced that he had changed the grade of an Indiana charter school founded by a prominent campaign donor while he was the superintendent of schools there.

Speaking defiantly at a news conference in Tallahassee, Tony Bennett, who was appointed Florida’s schools chief in December after losing a re-election bid in Indiana in November, referred to “malicious, unfounded reports” when he announced his decision.

The resignation came three days after The Associated Press published an article revealing e-mails that showed Mr. Bennett ordered his staff to change a “C” grade given to Christel House Academy, a charter school in Indianapolis, to an “A.”

The school takes its name from its founder, Christel DeHaan, a former executive at a time share company and one of Mr. Bennett’s largest donors during the 2012 election. According to the National Institute on Money in State Politics, she donated a total of $90,000 to Mr. Bennett’s re-election campaign.