Looking on the internet deeply has found these results:

former classmate is the most popular phrase on the web.

ex-classmate 1145,000 results on the web	former classmate 1690,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
My ex I'm an ex A young lady claiming to be your ex classmates. classmate is now the headmistress of a girls' school. Mike dropped out of school and started dealing to his ex classmate of your son. classmate wants to see you.	Lately I've been talking to some of my former classmates at Manor Hill. We have an exclusive interview with one of Twisp's former classmates. Taku, this is Okada, my former classmate. This time it was a former classmate of yours... I could go to the top dogs to buy quality stuff at a good price... because the local boss was a former classmate: I want to ask you about Charles and another former classmate of yours, Riley Wilson. I had a chance to meet one of my former classmates, Harold Laken. My self

ex-classmate

1145,000 results on the web

former classmate

1690,000 results on the web

More popular!

Lately I've been talking to some of my former classmates at Manor Hill.
We have an exclusive interview with one of Twisp's former classmates.
Taku, this is Okada, my former classmate.
This time it was a former classmate of yours...
I could go to the top dogs to buy quality stuff at a good price... because the local boss was a former classmate:
I want to ask you about Charles and another former classmate of yours, Riley Wilson.
I had a chance to meet one of my former classmates, Harold Laken.
My self