All of the schools must have the same "Begin school year date" and "End School date". This setting is located under Core > Settings > School Information > Years & terms > Year Setup. If your organization has more than one school (i.e. a middle school and upper school) these schools need to have the same start date and end date. Allow different start and end dates for individual schools, it would give us more flexibility to manage processes rather than using workarounds.
This setup affects all K-12 products including Billing Management. The school year is the default in Billing Management even when the billing year is different than the school year. It would make more sense to use the billing year as default for Billing Management.