The phrase "a former classmate at" is correct and usable in written English.
You can use it when referring to someone who used to be a classmate at a specific institution or organization.
Example: "I recently reconnected with a former classmate at university during a networking event."
Alternatives: "an ex-classmate from" or "a past classmate at".
James Wesley Harris, a former classmate at Creighton University, finally changed things for me.
In the fall of 2007, he married a former classmate at Stanford.
"Everything he did was stylized," recalled Jane Kleinman, a former classmate at the Rhode Island School of Design, where Mr. Talley took design classes.
"I know this is a momentous decision," said United States Representative David Price, Democrat of North Carolina, and a former classmate at Yale and Keohane's friend.
DR. MARTIN LUTHER KING JR., a painting by JACOB LAWRENCE, a copy of the "best Inaugural Address ever given" (Lincoln's second), a painting of Mr. Clinton by a former classmate at Georgetown.
MR. GOEL quit his job as an investment banker at Citibank in late 1999 after receiving a $1.2 million investment from Venky Harinarayan, a former classmate at the University of California at Los Angeles, and Anand Rajaraman and Rakesh Mathur.
So she enlisted Mr. Nagae, a former classmate of hers at Stanford, as her wingman.
Yet he was also a generous teammate; a former classmate of his at Harvard described him as a willing passer, "inclusive".
She described Mr. Pearson, a former classmate of hers at Hampshire College who majored in poetry and cultural studies, as both "smart and tough, but there's a side to him that's very sensitive and poetic that not a lot of people know about, and I think that slips out in every project he does".
(Boaz Weinstein, who is the founder of the hedge fund Saba Capital Management and also a former classmate of mine at Stuyvesant, told me, "I like his tactic: 'It's a bubble! Ride the rocket, baby!' ") Novogratz gathered some members of his staff to discuss the emerging fund.
"You could see his beliefs plain as day," said a former classmate of his at Randall K. Cooper High School, in Union, Kentucky.
