Old boys from one of Sydney's most prestigious public schools say they are disgusted that federal cabinet minister Scott Morrison will be a special guest at an alumni fund-raising event, warning it would be an embarrassment to their school to celebrate a man who has "so flagrantly disregarded human rights".

Almost 300 alumni of Sydney Boys High have signed a fiercely-worded letter to the school's Old Boys Union, which is organising a fund-raising cocktail event next month featuring Mr Morrison as one of its keynote speakers.

The invitation to Sydney Boys' High School cocktail evening with Scott Morrison.

Mr Morrison, the former immigration minister, is one of several prominent old boys who has been invited to reflect on how his life has been shaped by his time at the high-performing selective boys school.

But the invitation has infuriated many of the school's alumni, including the former Supreme Court judge Hal Wootten and veteran journalist John Pilger, who say it "is cruel and insensitive for the union to laud this man's connection to the school, its graduates, and their families."