10y as SEO1 for a HOD (which maxes out at SEO3, and should comfortably support SEO2) doesn't sound that great, to be honest - unless you've only recently taken on the HOD role. The fact that that P directly asked you about it might also signal something… If you really want to stay on in the current role and school, should probably be candid and perhaps check if there's anything more you should be doing.

However, just a thought - would you consider accepting the offer and shifting to an SEO1 role such as SH or ST? Your salary won't actually be impacted, and you can take the opportunity to refocus your energies…