The applicants must first cancel their existing interview waiver appointment if they want to apply for an emergency student visa.
After this, they must make a first-timer (walk-in) appointment. Only then can they request the emergency appointment
Before applying for an emergency appointment, the applicants must ensure that they have documentary evidence to prove their urgency.
Unreasonable applications have higher chances of being rejected
The purpose of travelling to the U.S. in order to attend classes in a timely manner can be seen as a genuine reason.
A list of documentary requirements for students and temporary workers can be found on the U.S. Embassy India website.