M 10 = refers to the date admission decisions are released by many prep boarding schools.
The 10 Schools Admission Organization is what I thought of first.
Probably includes:
St. Paul’s School
Groton
Hotchkiss
Phillips Academy at Andover
Phillips Exeter Academy
Milton
Deerfield Academy
The Hill School
Lawrenceville
Choate Rosemary Hall
One or two may not be in the M10. I do know that St. Paul’s School was one of the founding member schools (if I recall correctly, along with Andover & Exeter, but not sure).
M10 = March 10 = day of admissions decisions released.