You are making life more complicated than it actually is. Students in my classrooms NEVER habitually left their seats. Why in your classroom or school is it an issue, but in all my years of teaching it has never, ever been an issue with me or any teachers I know? How is that possible?
I will guess that you have not yet learned how to manage your classroom. There is no verbal warning, louder verbal warning, call another teacher, then send them to the office. That screams that you are timid, not in charge, and lack any real authority. Students will naturally take advantage.
When I have taught, any student that gets up for some unexplained reason, you ask them very directly, "What are you doing". If they have an actual legitimate reason, they will tell you. If they have any response that is other than appropriate you send them out of the class, send them to the office to wait, and then you personally go to the office and handle it after class. You don't have the principal or anyone else handle it, you handle it. Guess how many times you will have to send a student out of your class? Once if any. You will not have a second issue.
Note: I'm talking students past early elementary. By 3rd/4th grade students are more than capable and aware they stay in their seats unless an assigned activity,,etc. is taking place.