If you can handle Bio with Maths then it is fine but make sure that you are claiming this after getting yourself familiarized with the syllabus of both the subjects. In class Xth, I too like Biology but believe me, the Xth syllabus is not even 1% of +2 syllabus.
PE is probably the easiest extra subject because it requires you to write silly things like the dimensions of a football field & advantages of doing yoga, etc. You definitely don't require to study those at school. I took CS as my extra subject & got poor marks (Physics- 84, Chem- 85, CS- 69, for comparison) even after studying hard while my classmates in PE got above 90 by just running rounds around the school. I'm not forcing you to take PE, but if you join Aeronautics (or any engg. stream after +2 except CSE), then CS is probably just wastage of time & energy which you can otherwise devote to main subjects. Alternatively, if your school allows change in extra subject at a later date, you can opt for your choice now & change it later on.
Regarding coaching, if you can take the extra pressure, then it is beneficial but most of the time it harms more than it does good. As I said earlier, your first priority now should be to get the best marks in your board exam & make sure than your board exam marks are not affected by your coaching.