My perspective as a HOD (and former HQ officer):

The fact that you're doing a mandatory posting in HQ is a good sign already. Entry level EO roles at HQ are generally SEO1 'estab', so it's equivalent to doing SH/LH role in school. If you manage another B (or C+), think GEO5 would probably come soon enough.

That said, keep in mind that ranking at HQ tends to be more competitive (you are compared against other high-performing officers) and it would be harder to get B. Still, if your bosses value your work, your CEP might possibly go up another notch. You'll likely need to head some project teams while at HQ for this to happen, though.

If you stay on with MOE, after your HQ stint you are likely to be able to secure a SH/LH position when going back out to school (HOD is possible; but rarer if you've not been appointed KP previously).

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Just to comment on your previous 'B' streak in school - despite what your comment, I think that is a very positive sign, as it implies that your RO/HOD actively helped to push for that grade, ultimately with the endorsement of school leaders. Even if it might not have seemed like P had taken notice of you, he/she clearly supported designating you as high-performing staff.

If CEP had been raised to SEO1 level, this is another good sign. Teachers start with GEO5 CEP by default. Someone had to actively suggest your CEP to be moved up - and P would have had to endorse it. Very few non-KP (maybe 1-2 in a school) would have CEP higher than SEO1. Even SEO1 CEP would not be all that common.

In summary: if you truly enjoyed working as a teacher, please don't be discouraged by what you perceive as poor career advancement. I think you're doing pretty well already!

p.s. If you're feeling discouraged in the future and feel comfortable enough doing so, I also suggest having a frank discussion about these matters with your RO. I learnt a lot discussing such matters with my DD (a former VP). Recognise that this depends on the type of ppl your bosses are, of course.