After they write their initial understanding, I show them The Science of Static Electricity from TedEd. I then ask a few students to share what they think static electricity means, and I have them write a working definition in their notebooks, as well as write static electricity in the index of their notebooks.
I then show them the same demonstration again, and ask them what they could change about it. I listen to a few ideas, which helps students that might be struggling to come up with something, and then ask them to write one experimental question they would like to explore further. This could be the size of the balloon, the length of the hair, the material of the scraps, the number of seconds the balloon is rubbed, etc.
I do try to approve each question before they begin testing, and require them to make a prediction about what will happen, and collect data in their notebooks. For management, it may be easier if you have them all turn their notebooks in, then call them up one or two at a time as they work on something else independently. I do allow partners to work together if they are interested in the same question.
When their experiments are complete, I have them go back to their tables and share their results with their tables, and write a conclusion based on their results. Drawing conclusions without a really clear graph can be tricky for them, so I ask them about patterns, or rules they think they could make about other things, even if they haven't tested them yet.