Nice concept, it will be helpful to student and worth to develop.
I'm native Android app developer with 3.5 years of experience. And bases on my experience it seems project can be achieved with all requirements.
Time estimation:
1) App design, Subject entry and track individual session with time and additional notes. (16 hrs)
2) If we provide, edit option for each session, I guess it should be there (8 hrs)
3) Basic listing of all sessions and filter by subject and date (12 hrs)
4) Chart representation (24 hrs), it might take more if we add consider more details and many variations
5) Reserve hours for any modification / additional and testing (8 hrs)
It seems 8-9 days work, but I can manage only 20 hrs / week, so it might take me to finish till end of this month, but I try my best to finish in your deadline.
Regarding costing, I've consider best possible bases on requirements. If you wish to continue, we can have a talk and even you wish to implement in your budget, we can alter requirements slightly which fit best.