Seen a number of times with different departments.

Some subjects like Art, Music, CPA, FCE, DnT, which are compulsory subjects but somehow have very small headcounts of around 3 teachers. When 1 leaves, the remaining 2 are in big trouble. When 2 leave, the sole remaining teacher is forced to leave too, because there's no way one person can teach the entire school. Hiring FAJTs doesn't help much that teacher much (if there are even FAJTs available to be hored in the first place). In such cases where the entire team of subject teachers left and no FAJTs could be hired, got see schools resort to outsourcing the subject to vendors.

Also seen situations in which "additional headcounts" are mustered by redeploying teachers to teach their CS2s and even CS3s to plug manpower gaps.
It is sad to see teachers being asked to suddenly teach subjects they haven't taught for over 10, 20 years.

Ps and HR can also interfere with posting, and hold back people to meet manpower needs.

Anyway, don't let such concerns hold you back if anyone is planning to leave service.

The official stance from HQ Is that teachers are in excess, and there's too much pride in the moe leadership to admit that they are wrong, and schools are understaffed. Someone probably received awards and promotions for implementing lean management practices in MOE, the new trendy buzzword in management, conveniently ignoring the fact that unlike the private sector, public services cannot afford to fail, cannot afford to close down due to lack of manpower.

As long as moe leadership continues to insist that teachers are in excess, not even acknowledging that there is a problem, headcount woes will only worsen.