Ratta maarna is basically mugging up something. It's an essential activity of almost entire schooling system in India, and in fact is required in most colleges as well. Student of The Year's song Ratta maar seems to be trying to take the benefit of the same while talking about what every student considers a major problem.
The words Ratta maar, zor se, literally mean mug up, with (all your) might.
Check All Lyrics, Meanings and Translations from Student of The Year HERE.