What U.S. high school is recognized as having the largest alumni association of any high school in the nation? When was it established and by whom?

As of March 2006, Lane Tech Alumni Association has over 5,900 active members all over the world. As a result, the Lane Tech Alumni Association has the largest membership of any high school alumni organization in the entire United States.

The Lane Tech Alumni Association was established through the efforts of the January and June classes of 1932 and their involvement in the school's Diamond Jubilee celebration in 1983.

The "who" is not something I have yet found.

https://en.wikipedia.org/wiki/Lane_Technical_College_Prep_High_School

https://en.wikipedia.org/wiki/Lane_Technical_College_Prep_High_School

The Lane Tech Alumni Association traces its roots back to the efforts of Roger Paulsen and Joe Bozovsky, back in 1972:

The Lane Tech Alumni Association traces its beginnings to 1970 when Lane alums Roger Paulsen and Joe Bozovsky from the Class of 1932 began planning a 40-year reunion to be held in 1972. Prior to this, no formal reunions had been held and the school could only furnish four contact names from the class of 1932. The reunion was held and that motivated a core group of alumni to make it an annual event. Two years later, alumni from other classes were invited to participate in not only a yearly reunion but a series of regular social events. Group activities continued to gain momentum and in April 1977 a state charter was obtained for the "Lane Tech Class of '32 and Associated Alumni". Years later, in an effort to become more inclusive of all graduates, an amendment was made to the state charter renaming the group the "Lane Tech Alumni"

In 1984, Principal Norman Silber reached out to alumni to assist the school in planning the 50th anniversary of the opening of the "new Lane". An office, across from the Main Office in the school's foyer, was assigned to the Lane Tech Alumni Association. Volunteers began the task of searching phone books and the media to locate graduates in order to create a database. Two years later, spearheaded by Bernie Joseph, class of 1940 and LTAA President, the Alumni Association was incorporated as a not-for-profit in the State of Illinois.

https://lanetechalumni.org/about-us/alumni-association-history/