Answer:
well you could have a test group where they don't have phones(independent variable or control group) and students (of any age) who have.
but the dependent variable (or the one being changed) is the test grades of students. measure the grades of the students with-out phones and ones who have phone.
Explanation:
if all goes correctly, the average of the people who use phones should be relatively lower than students who do not have phones (never mentioned range because some students can have the highest grade out of all the control group and still have a phone)