The key to becoming an efficient physical educator is to use all the time I have properly. It may sound easy, but with 20-30 students running around in a small gym, it may cause some unexpected things to come up. Unlike other subjects, physical education is different between school districts. The time you are given with your students, is all the time you have to make sure you get your point across. Being able to transition from different activities will help keep the students moving and not allow the students to get side tracked. Personally, I like to write in depth lesson plans stating exactly how much time I will spend on each activity, like I had done in one of my classes. As I learned quickly, the time you have set, doesn't always go as planned. Having backup ideas in your lesson plan will be reassuring just in case the lesson gets out of control.
Having other options like a whistle, scoreboard, stopwatch, etc. will be good to start and stop activities going on. This is not just about physical education either, I'm talking about all subjects. Lot's of time during classes are spent doing miscellaneous things that are to benefiting the student. It may be easier to me to talk about it because I currently have a sister in 8Th grade. I get to see her schedule and talk to her about other things going on inside the school. Many teacher's often ave there lesson and expect the student's to learn through doing homework or other assignments. Unfortunately there are more than one way to learn (visual, auditory or experiential), and the teacher should try to accommodate the different styles, In the gymnasium this may be a little tougher, but bringing in visual aids and videos can help.
Going back to the physical education perspective, time in the gym is the most crucial thing a teacher must keep in the back of their mind. For example, if your lesson plan says your spending a certain amount of time on this activity, then switching, but you never switch, you could be liable if it came down to it. Making sure you stick to your lesson plan, and make reasonable times for each activity and transition. It's easy to forget about transition time while writing lesson plans, but having a general idea will keep the times as realistic as possible. Whatever setting you are in (inside or outside), have a plan, and stick to it.