The question

I made a payment in the amount of $1,800+ through my checking account. My bank had flagged the payment because it was a large amount and I never had a payment that large before being withdrawn. I waited for the balance to show up in my student account but it never did. I just went about my life not thinking too much into it. Almost two years later, my school sends me a certified letter saying that i owe them immediately for an invalid payment they didn't catch. Rightfully so, I do owe them, but they're not letting me set up a payment plan or anything and they said I cant attend their school unless I pay the money in full. What can I do about this situation? I didn't even know a school could do that. I'm so close to graduating and I need to finish this semester.