This is very common. A lot of schools take on NQTs on one-year temporary contracts and state "curriculum need" as the reason for the post (someone needs to teach the classes). If the school roll numbers fall, or there is less demand for your subject at KS4 or KS5 (from a secondary point of view) then your job is no longer needed. If you like the school and want to stay then apply for it. But don't assume that they will give it to you. If they liked you that much they would offer it to you without advertising externally (they don't need to do that by law, or at least I don't think they need to). Someone with more experience may be able to advise further about where you stand legally.