I usually go through this as a demonstration lab. I have three strings hanging from the ceiling in my classroom: two the same length and one shorter. I first explain the importance of a controlled experiment to my students (that we can only change one variable at a time, or we would not know which variables were responsible for the change) and show them the control set-up: a 200 g mass hanging from one of the longer strings pulled back about 45 degrees. Then, I release the pendulum and we time how long it takes to swing back to my hand. We identify that as the period: the time for a full cycle. I like to graph this motion (a basic cosine wave) to show them how a swinging pendulum is related to waves. I then ask them if that period will change if I put a smaller 100 g mass on the end. We draw the picture in that box and I have them write in their hypothesis. Once they have all guessed, and we have taken a quick poll of the class, I pull them both back (same length string, same angle) and release simultaneously. Leaving my hands exactly where they were, the kids see that I catch them at the exact same time, as they returned to my hands at the same time. We repeat this process for the shorter string and shorter angle. Afterwards, we discuss gravity (and how I'd love to prove it to them, but I simply cannot change gravity!) and how it also affects the period. They then write their summary and I grade them on including the two things that do affect the period of a pendulum and the two that do not.