Based on the search results, there are several things that schools have once a year. Here are some possibilities:

• Year-round schools: Instead of having one long summer break, year-round schools break the academic year up with several medium-sized breaks throughout the year.
• Standardized tests: States must test students in reading and math once a year in grades 3 through 8, as well as once in high school. They must also test kids in science once in grade school, middle school, and high school.
• Academic years: Every eligible program must have a defined academic year. A school may have different academic years for different academic programs.
• Start time legislation: Bills related to school starting hours have been introduced in at least 26 U.S. states and territories.
• City Year commitment: City Year and their partners commit to multiple years of service in schools so that students “grow up with City Year”—their seven-year promise.

Its worth noting that some of these things are specific to certain types of schools or certain regions, while others are more universal.