While I don’t know your principal specifically, I've had great experiences with my principal and superintendent while implementing flexible seating in my classroom. This is definitely a new concept and my administration was curious to see how it would affect student learning and engagement. I would recommend doing lots of research on how to manage flexible seating and be prepared to learn as you go. Have behavior management strategies in place before telling your students to just "pick any seat today!" You must know your students well before setting this in motion, I.E. who shouldn't be near whom, chatter pairs, or high behavioral students who need a "controlled" flexible seating plan. It would help to start with a small block or subject first and then work your flexible seating plan for longer periods throughout the day. I would recommend creating a chart of expectations with your students and place it where everyone can see and reference it. If your principal comes in, he/she can also read the expectations and see if your students are following them. Have clear cues for getting students' attention and for getting them into organized groups for transitions. If you can demonstrate strong behavior management, knowledge of your students, and student engagement, I don't see why your principal would frown upon flexible seating.