Then it is still tied to the previous owner. Did the seller disassociate it following these instructions?
https://support.apple.com/en-us/HT201065
And, since this was used in a school district where there was possibly one Apple ID (the IT or admin) for all the machines, it needs to be disassociated from that.
Also, did you reinstall the original OS of the machine (not the one it came with when you got it)? It would help if we knew the model/year of the machine as that determines how to reset it to factory condition.