Joseph
after approximately 10 years-or more- of teaching many rabbeim tend to transfer to becoming principles or assistant principles (based on their experience but also due to their knowing how that school they have been working in runs) this of course is only due if the school is growing more classes & therefore needs more principles
i.e. not all schools grow. some stay 1 to 3 classes per grade & that’s it after that there’s no more room for more growth because they do not want to grow, as it involves more then just finding a new rebbe for the additional class per grade in the years ahead.