If you think you are being groomed as a KS3 specialist and don’t like the idea then I’m still of the opinion that you accept it is happening, but say to yourself, one way or another I’ll be back teaching KS4/5 for 2019-20. And it might be you have to look our for another role. Science teachers are not in abundance. You’d be welcomed by another school. Sometimes you have to attend an interview and then 'decline the job' just to rattle the bars. Something along the lines of, 'I don’t mind doing it for one year, but understand it is so important to me that I get KS4/5 that if this isn’t going to happen then I’ll probably look somewhere else. I’m just being candid and honest here about how important it is to me. But I do understand that you need me to do KS3 this year.' I've a good track history of telling senior bods that I am being honest/candid with them and telling them things straight before telling them something they don’t like to hear. It is a better non confrontational way than hidden passive aggressive stuff.