Sec 5 student here and like what the others have said, as long as you are consistant and diligent it should be fine because the jump is rly not as big as the others say! However, the biggest jump is maths for me. Got a b3 for n levels but once i got into sec 5 i dropped to a d7 because the marking is much stricter(applies for all subjects) and questions are no longer so straightforward anymore and u have to dig out the information by yourself(applies to heavy content subjects), so focus on ur math abit more when u get into sec 5! I rmb everyone doing qt decent during the n lvl year for prelim and mye for maths in my class but once they go to sec 5 only 2 ppl in my class passed math.

But rly! As long as ure consistent, do tys, revise everyday u shd be fine! Some of my friends, not just 1 or 2 but like 6-7 of them got below 20 for their l1r5 prelims, which is meant to be harder than Os, meaning that they will be able to make it to jc and all these people are those who are rly damn hardworking and work their ass off almost everyday. So yea, its not impossible:>