Final answer:
The Spanish subject pronoun a teacher would use to refer to her entire class is 'Ustedes'.
Step-by-step explanation:
The Spanish subject pronoun that a teacher would use to refer to her entire class and say "You guys need to complete the assignment" is Ustedes. In Spanish, 'ustedes' is the plural form of the formal subject pronoun 'usted', which is used to address a group of people in a respectful manner. It is equivalent to the English pronoun 'you all' or 'you guys'.
Learn more about Spanish subject pronouns