First, make sure it is the IRS. However a CP2000 Notice received in the mail, in an official IRS envelope, should be legit.
I would provide as much detail as possible. Copies of cancelled checks, bill and/or receipt from institution for the tuition payments.
Prepare short statement naming the institution, explain how payments were made, and date and sign. Attach copies of any and all documents that show what the tuition was/is, how it was paid, and if you have a receipt showing paid, include that as well.
Send copies of your documents, not originals....keep copy of your signed statement and send the original of that.
I recommend mailing with return receipt requested...proof you mailed it and they received it.