Pelham Memorial High School will hold its Centennial Commencement Exercises tomorrow at 9:30 a.m. on Franklin Field with the graduation of 240 students. One hundred years ago, PMHS held its first commencement with a class of 19 seniors. Never-before-seen materials from the class of 1922, recently donated by the daughter of one of those graduates, can now be seen on the Pelham Town Historian website, which also tells the story of that alum and provides information on many of her classmates.