Today, it would be 'I am in year nine'. When I was at school it would have been meaningless - the highest form was the sixth form; the 'sixth form' was divided into two years - 'Upper Sixth' and 'Lower Sixth'. People in either year were called 'sixth formers'.
PS Sometimes it could last a little more than two years. Because entrance exams for Oxford and Cambridge used to be taken in December, pupils aiming for Oxbridge sat the exams in their 7th term rather than their 4th.