There is a micro-phenomenon of new wave, special snowflake directors that try to revolution the canons of classical opera. I saw Carmen being banged behind a burning car next to what looked like a flamenco-infused Thunderdome. That was in Paris opera, stronghold of modernist SJW nonsense.
Here, the expression is a shorter version of matriculation college. In a number of Canadian cities, many government-run secondary schools are called collegiates or collegiate institutes (C.I.), a complicated form of the word college which avoids the usual post-secondary connotation.
Usage note. Alumnus (in Latin a masculine noun) refers to a male graduate or former student; the plural is alumni. An alumna (in Latin a feminine noun) refers to a female graduate or former student; the plural is alumnae.