My students already know that this assessment will be a team challenge problem, so they're ready at the beginning of class for me to use popsicle sticks to randomly sort them into partners. They also know where all the supplies/materials they might need are, like graph paper, rulers and calculators. My video explains why I chose to make this a team assessment and how I know my students really get it if I'm not assessing them individually. If you have a large class or don't feel that a team assessment works for you and/or your students you can still use the challenge for individual students but have each student just complete one of the two problems.
I distribute the Assessment Challenge after assigning partners and while my students are working I walk around giving encouragement and redirection as needed. I tell them when there are five minutes left so they have time to wrap up their work.