Because, there is a must condition over there, if you want to say "I need to be the best student", we can translate it as "En iyi öğrenci olmam gerekiyor". It's difference between must and need to in Turkish (need to-gerekmek) :) I hope it helps.
So how do you differentiate between should and must?
'Must' conveys a stronger meaning than 'should'. So is there a way to distinguish this in Turkish? Or do you just use zorunda for clarity if needed?