Incredibly unfair and unjustified

I am a teacher. Before my first baby I had a teaching and learning responsibility for a whole school issue. When I returned on three days I was not allowed to keep this because ‘the role requires a full time presence in the school’. Five years down the line and no one has the role and it has never been advertised as a job role. I challenged the decision at the time but decided it wasn’t worth my time or energy. After my second baby I retained my 3 days as a normal classroom teacher. However, an opportunity arose for an additional responsibility within my department; basically my head of department retired. I was interviewed and appointed to this role but they changed both the title and salary because I am part time. My head has implied in meetings that I would gain both the head of department title and financial reward for it should I go full time despite the fact that my job role and responsibilities would not change at all from what I am currently doing on three days a week. Incredibly unfair and unjustified in my eyes but I feel completely like I am in a catch 22 situation- if I give up my current role and stay as a standard teacher I am demoting myself but if I agree to increase to full time hours all I really gain is more classes to teach and not more managerial responsibility.