depends on the weighting of assessments, how far you are behind in marks, performance of others in the grade, etc.
for example, lets just assume you have done 50% of your assessments, and had an average mark of 70, whilst those in the top 5 had an average mark of about 85, and we will just ignore all of the other students. now lets say that the top 5 maintain their average of 85 in the rest of their task, which means that their overall mark would be 85. so now you need to make your average mark up to around 85. so this means that, where x is your mark that you need to get,
0.5x + 0.5*70 = 85, which gives x =100. so to catch up with your peers, you would now need to get 100% on your remaining assessments. this is ignoring a couple of factors but it can be difficult. in the real world, let's say that a few people slip up, and the person at rank 5 gets an average of 75 overall. now you would only need to get 80% on your remaining assessments. so it all depends on previous marks, how those in your cohort perform in the future, and also sometimes how your school calculates your rank (in some instances i believe a few schools just average out your ranks in each task instead of the marks, not too sure).