O/T I am so angry and upset

I work in a secondary school. A lady is retiring July 2010. Her job is great and i sort of do part of it and i am good at it. I have been told to apply for the part time job with her 15hrs a week and then when she leaves i would take it over. everyone agreeing and because of 1 or 2 people i have been told that i cant unless i resign from my post and just work the 15 hrs a week. it is unfair as i am the earner, hubby house husband.
would anyone know if i could claim benefits, i dont think that i could as i would be leaving a full time job.