Trading with Schools was infiltrated by malicious software on 21 September, with the authority becoming aware of the attack four days later and shutting down the system.
The platform is used by 88 schools under the council’s umbrella, with 49 of those schools only purchasing the provision of internet services.
The remaining 39, the schools whose data has been published online, rely more heavily on the system for day-to-day tasks such as scheduling, lesson plans, recording staff details and payroll information.
The platform can also house sensitive information about children, including educational psychology files.
Since the system was shut down on 25 September, many schools have been unable to use their phones and teachers have been faced with longer working hours as they struggle to find ways to deliver their lessons.