The Independent School League is the oldest Independent School athletic association in the United States. The ISL is composed of sixteen prestigious New England preparatory schools that compete with each other athletically. Schools are noted for their academic rigor, traditional rivalries, and excellent college placement. Collectively, these New England schools are sometimes termed St. Grottlesex. The term is a blend, using the St. part of St. Paul's, St. Mark's, and St. George's, part of Groton School, combined with part of Middlesex. The Independent School League schools include:

• Belmont Hill School (the Hillies)
• Buckingham Browne & Nichols School (the Knights)
• Brooks School (the Bishops)
• The Governor's Academy (formerly Governor Dummer Academy) (the Governors)
• Groton School (the Zebras, unofficially)
• Lawrence Academy at Groton (the Spartans)
• Middlesex School (the Zebras)
• Milton Academy (the Mustangs)
• Noble & Greenough School (the Bulldogs)
• Rivers School (the Redwings)
• Roxbury Latin (the Foxes)
• St. George's School (the Dragons)
• St. Mark's School (the Winged Lions)
• St. Paul's School (the Pelicans)
• St. Sebastian's School (the Arrows)
• Thayer Academy (the Tigers)

Traditional Rivalries

• Belmont Hill vs. Governor's
• Brooks vs. Lawrence
• Buckingham Browne & Nichols vs. Roxbury Latin
• Groton vs. St. Mark's
• Middlesex vs. St. George's
• Milton vs. Nobles
• St. Paul's vs. Rivers
• St. Sebastian's vs. Thayer

Unofficial Rivalries

• Governor Dummer vs. Lawrence (boys' hockey)
• Brooks vs. Nobles (boys' soccer)
• Middlesex vs. Roxbury Latin (football)

Private schools | Independent School League | High school sports conferences