DMM/Fanza (the adult section) sells each game digitally. Use a VPN set to Japan and a credit card that allows foreign transactions.
To understand the demand for a download, one must understand the product. Developed by 0verflow, Schooldays is a visual novel that initially released in 2005. Unlike many visual novels of its era that relied on static sprites and text boxes, Schooldays was revolutionary for utilizing fully animated scenes, giving it the feel of an interactive anime. SCHOOLDAYS SERIES COMPLETE BOX Free Download
You cannot legally get the as a single package. But you can assemble a near-complete experience for zero dollars by combining free trials, demos, and official promotional releases. DMM/Fanza (the adult section) sells each game digitally