College rector Michael Bongers alerted the 21 students Thursday their readmission had been declined, the Sydney Morning Herald reported.
The expulsions stem from an incident in March in which 33 students pressured a female student to drink a toxic cocktail comprised of shampoo, alcohol and dog food. The victim was later rushed to a local hospital with a bleeding stomach.
Twelve of the 33 students involved in the incident did not reapply at the college.
Following the incident, the Catholic Archbishop of Sydney, Cardinal George Pell, and the university Vice Chancellor Michael Spence ordered five fellows on the college's governing body to quit. Within two weeks, all 18 fellows had resigned.
Pell replaced the board with 10 lay fellows and six clerical fellows who will govern the college for three years.
The students who were kicked out of St. John's are allowed to enroll at the University of Sydney, however they are unlikely to be admitted to other religious colleges situated on the university grounds, a university official said.
"The colleges share committee members, they share ideas and information, they are quite close in that respect," the official said. "These students would ultimately need a good reference, and it goes without saying they won't be receiving one from St. John's."