Class sizes are determined by the New York State Public High School Athletic Association (NYSPHSAA).
Classes are determined by the number of students in the school. The official NYS BEDS (Basic Educational Data System) numbers are used as the official count. The majority of sports are governed by a single set of "cut off " numbers. Lacrosse along with football, volleyball and Cross Country are governed by a different set. Starting in the 2009 season the numbers were changed to the following
For the 2011 season the cutoffs for Lacrosse are as follows:
| Class A||1050 & up (was 1100 and up)|
| Class B||650-1049 (was 650 to 1099)|
| Class C||0-649 (no change)|
Moving on up
Procedures exist to allow schools in a smaller class to move up to a larger class. in the summer of 2005 the Section Five Executive Committee adopted a new policy that generally requires teams to announce their intention to move by July 1st (for spring sports).
School teams can petition through the Section to combine with other schools when they can demonstrate the need. For example, Pittsford has, for many years, combined students from Mendon and Sutherland for one district team. Whereas Sutherland and Mendon are individually Class B teams, as a combined team they are classified a Class A team.
Single Gender Schools
Schools that serve only one gender (like McQuaid) have their student BEDS numbers doubled in order to determine their Class position.