You should plan your time to avoid multi-tasking. Focus on one activity at a time, do it in the right place, with the right mindset; do it quickly, and do it well.
In order to make this work, you may need to consider proceduralising tasks that you do repeatedly. Proceduralisation is an ugly word, but an important tool for an efficient teacher. It means doing things in the same sensible way each time, and turning them into a routine which can be done without thought, without worry. This doesn't mean that you teach without thinking; on the contrary, you should be constantly mindful and completely present in the classroom.
Routine non teaching stuff, though, should be made routine – and stress free. Proceduralisation is explained a little more here.