It is likely that students will need to be redirected if they get off-task. Don’t assume kids who get off-task are bad kids or not interested in CS. Check with the teacher before class to ask how they would like you to interact with students who are off-task. They might suggest that you introduce yourself and ask "What are you working on?"
Example: "How would you like me to respond if students are off-task?"
See this and related tips as a Tip Sheet: http://csteachingtips.org/Tips-for-classroom-volunteers