Schools competing under the NHIAA are grouped into four classes:
Division IV: up to 350 students
Division III: 351-670 students
Division II: 671-1200 students
Division I: over 1200 students
Schools are reclassified every two years. The classification thresholds are periodically adjusted so as to result in an approximately equal number of teams in each class. Schools may petition the NHIAA to play in a class higher than that in which they would otherwise be placed.
For some sports (e.g. football or ice hockey), competition is not organized based on school class, but based on divisions established by the NHIAA committee governing the individual sport.
Classifications for 2006-2008 are as follows (an asterisk indicates a school that has petitioned up to a higher class):