Answer:
Independent variable
Step-by-step explanation:
Always remember that when you are testing something in an experiment that something about be considered an independent variable. If you're measuring something in an experiment that something would be considered a dependent variable. In this case, you are stating that second-grade students are better at jumping rope than first-grade students which can only be found out by a test. You're testing to see who jumps better second graders or first graders therefore the students are considered to be the independent variables.
I hope this helps.