Rather than suggesting some apps, I’d suggest you that nothing is tough if you work hard for your aim.
It is not tough to fetch marks in 12th standard. All that you have to do is keep yourself focussed throughout the entire year and study hard. Solve previous years questions and revise your books well. Find out the topics that you are weak in, and devote more time to understand them.
Also, one tip that I’d like to give you from the examination point of view is that keep a check on your time. Alot of students are unable to keep a time record and so loose out on marks.
Hope this helps! 🙂