some medical schools have a requirement stating you need a B in your 4th As level... but thats only some. When you say "does it matter what one you drop", depends what unis you are applying to... e.g. not all require Biology up to A2, some (but few) dont even need Chemistry up to A2 (but will require a good mark at As).
Lecture wise... dont worry about it. It will probably just be all about the October deadline, and getting lots of W/E, and getting good grades etc. I spent so much of my time last year going to extra curricular lectures out side of school hours etc, and didnt even put them in my ps because there wasnt room... however i guess they are good to mention when it comes to an interview if you learnt some interesting stuff. But all in all, i wouldnt worry about it... it'll all be info you can find on the internet or will be told again at a later date.