Does your secondary school only have 40 students or so?
In the us, a ‘class’ is all the students who will graduate in a given year, such 2019(for instance). So, your rank in class would be all students from Year 9 in 2015. Compared to them, are you a student in the bottom 30%?
In the US, a student ranked in the bottom 30% of her school wouldn’t be going to college: do you attend a prep school where everyone goes to college? (Is that rank your own estimate or does your school officially rank you?)