With all systems, we need to start with our genre. Fantasy and even dystopian SF might have much poorer infrastructure than modern Earth, while other SF might have the same or greater. This and how established the government is will impact that society’s ability to create a system, so consider whether it’s a new sovereign power or one of a several decades or more. While some systems extend throughout the society, some are more localized, so determine the scope of the system that’s being invented, such as if it’s a city, state, or country. These decisions are the broad strokes we need. Details such as when school starts and ends, or what kind of money is used and how it’s configured, are more a personal preference for our setting and we can make at any time based on what we like more than what’s likely.