I would contact the school IMMEDIATELY as this would suggest that an incorrect assessment mark has been sent through to the BOS (schools send in the assessment marks not the ranks).
If they say it is correct then sit down with your teacher and check that have the same marks as you for yourself and ask them to check the other students' results as well as the error may be there. Please, from a teacher's point of view, talk to them first - we are human and make mistakes and will correct them (well I certainly will) if they are wrong. Don't go off half-cocked though.
If you still aren't happy you can appeal to the BOS and have them investigate your assessment mark and rank with the rest of your class.
Schools are allowed to tell students their final ranks only when they have finished all their exams according to the Modern History syllabus (which is what I teach). My school issues them with a statement of their ranks in each subject after their final exam if they wish to go and collect it. This year, so far, about 10% have done so!!! It seems that they trust us.
Teachers should NOT be giving you your final assessment mark as this could cause problems when the scaled assessment marks are issued with your results.