Thank you for clarifying my doubt. I have one more query- is this question correct? "Usen't you to go to school with him?"
Note the spelling. It's not "usen't", it's "usedn't" because it's a contraction of "useD you not".
I would say that the sentence is OK, but as I said, it sounds very old fashioned. My grandfather, who is 90 years old, might still use this construction but I don't know anyone younger who would say it.
You used to go to school with him, didn't you?
Didn't you used to go to school with him?