Answer:
The null hypothesis would say that class size has no effect on students learning.
Explanation:
Null hypothesis is a type of hypothesis that is used in statistics which proposes that there is no difference between a certain characteristic of a population or the data - generating process.
Now, in this question,the school district is considering increasing class size in the elementary schools but we want to check whether larger classes may have a negative effect on students learning.
Thus, the null hypothesis would reject this claim of larger classes having negative effect on students and say that class size has no effect on students learning.