Ucas predicted

My sixth form has a "Final UCAS Prediction" deadline where they say that it is final and can't be changed, however one of my subject teachers told me that if I impress in my next mock it can be changed, whereas another one of my subject teachers told me that he doesn't believe that predicted grades can be changed now. Can it be changed until I send my application to universities?

That is entirely up to your school - every school/college does this differently, and so you need to speak to whoever organises UCAS applications at your school.
Just be aware that you cannot change predicted grades after an application has been submitted - ie. you could wait until January to submit your application if you need to.