For students entering in year 10, agriculture is compulsory in year 10. Though you are supposed to do agriculture in year 11, most new students do not choose ag on their subject selection form. The teacher in charge of this does not check whether a student is new or not new, so this is overlooked. (ie I know lots of students who are new in year 10 and don't do ag in year 11). Apparently the school is making ag compulsory up to HSC for all students from now on. However, I'm not sure if this affects your grade.
With regards to your interview, you will do fine. As long as you are respectful and don't say something stupid (eg. I want to go Hurlstone because it is close to the station, I don't like agriculture - I just want to come for good scaling) you will be fine.