This cannot be fixed, since the administrator of the device probably a School has tied the device´s serial number to their network chances are it´s also stolen but anyway. The only 2 solutions to this are Attempting to change the device´s serial number which is pretty much impossible without a certain set of skills and if you don´t have any of these skills you will probably break the device. The other solution is to contact the company or district that owns the device to untie the device.