If a GEO3 does a HOD's job well, they certainly are in the running for an A, but there is really no 'rule of thumb' for As.. it depends on how competitive the school's staff are.

I've seen non-KP colleagues who never dabbled in much management work at all getting A because of excellent work on some high-key projects (things like new CCA establishment, major school milestone events with VVIPs attending, coaching fellow teachers to support a major syllabus revision, etc.). These are not necessarily the role of HODs.

If you read the KRAs for GEO3 especially, the 'A' column doesn't really expect the officer to be a high calibre middle manager - only a good experienced teacher (perhaps on par with his/her super lao jiao GEO5 colleagues).