"It was a horrible mistake. We regret it deeply," district spokesperson Michael Evans told ABC11 Eyewitness News.
The computer error printed student ID numbers on the cards which were not mailed in envelopes. For a small percentage of older students, the ID number is also their SSN.
"That came to our attention yesterday, and we began immediately going back through the process to identify which families were impacted by this," said Evans.
Officials say the numbers were not identified on the cards as Social Security numbers - which may help.
"We believe the information was exposed, not compromised, frankly. You would have to know exactly what you were looking for," said Evans. "It doesn't appear to be a three-two-four digit sequence like most people are used to seeing on a social security number. It's just nine numbers."
The district is now offering affected a year of free credit report checks. It's a $20 value per family.
"Which is significant sum of money, but it's a significant mistake. And we want to make things right for the families and give them some peace of mind," said Evans.