Requirements for teaching are different from requirements for Graduate admission.
Graduate students who wish to be considered for teaching assistantships must provide evidence of spoken English language proficiency, regardless of country of origin, nationality, or the language of instruction in any high school, undergraduate, or graduate degree program they may have previously completed.
Exempt: Students who have continuously attended English-medium schools since age 13 in Australia, Anglophone Canada, Ireland, New Zealand, the United Kingdom, or the United States (except Puerto Rico) are exempt from the English proficiency requirement and are cleared for teaching.
Not exempt: Students who completed an undergraduate and/or master's degree at Illinois or another US university are not exempt. Nor are students who were exempted from providing a TOEFL iBT or IELTS score for graduate admission.