Here's why:
* "Eres" is the informal "you are" in Spanish, used for one person (tú).
* "Ustedes" is the formal "you" used for multiple people.
To make the sentence correct, you should consider these options:
* If you are talking to one person:
* ¿Eres estudiante? (Are you a student?)
* If you are talking to multiple people:
* ¿Son ustedes estudiantes? (Are you all students?)
Remember to choose the appropriate form of "you" based on the number of people you are addressing and the level of formality.