Answer: 20 names from each grade
Explanation:
The two answers it comes down to are between every student from all of her classes and 20 students from each grade, however since the poll wants to determine how many people across the entire student body would attend, pulling from just Miranda's classes would add a grade level bias to the dance and would not be entirely random. 20 students from each grade is completely unbiased and properly represents the entire student body.