A History of Sigil Prep
Pre-Foundation
Several decades ago, a legendary Fighter from the World of Greyhawk, Robilar, sought to pass along is knowledge to a new generation and opened the Chainmail Academy for Adventurers, where he taught both practical fighting and tactics. He gathered a few students, gained a bit of a reputation, and after a few years and three graduating classes, his friend Bigby came and made him an offer to expand his school to include teaching magic.
Shortly, the pair of them opened the Greyhawk Academy for Adventurers, often referred to as the "Original" Sigil Prep. The first curriculum was rather vague and poorly organized, but y'know. You graduated if the faculty appreciated your work. At first, only Human, Dwarf, Elf and Halfling students were accepted, though "theoretically" anyone could attend. There just weren't any programs for them.
GAA began offering programs in Paladin, Assassin, Druid and Monk but these were firmly included as options in the Fighter, Thief and Cleric majors respectively. These would be shortlived at Greyhawk Academy, but would become mainstays at Sigil Prep.
First Curriculum
After a few years, Bigby decided a more defined curriculum was needed, and also hoped to entice a wider range of students to the school, so he arranged to buy a pocket dimension within the pocket dimension that is the City of Sigil. He began construction on what he deemed the Sigil Preparatory Academy (despite it being a University and not a Prep School). Robilar, meanwhile, chose to remain at the Grayhawk Academy, which taught a more Basic curriculum, while Sigil Prep was considered to be a more Advanced program. Robilar and his faculty streamlined the GAA programs a great deal, while Bigby intently worked out a more detailed program for Sigil Prep.