An image forming system which, even when overprinting is made on a sheet processed by an external device, prevents misregistration of the image formed on the sheet, and a control method for the image forming system, and a non-transitory computer-readable recording medium storing a program. The image forming system includes an image forming device, an image reader, an arithmetic processor, and a controller. The arithmetic processor calculates the information on shrinkage which occurs in the sheet when the sheet is subjected to a treatment process by the external device, as external shrinkage information. The controller calculates an image correction value according to the external shrinkage information. The image forming device forms an image according to the image correction value when forming the image on the processed sheet.