For Bio and chem yes but with a lot of exam practice and time management I think you will be fine Also the challenging part I guess for you this September is all the university applications and interviews and personal statements to do on top of mocks Make sure all the topics you learnt for year 12 you are confident in and if you have some topics you are struggling with please speak to your teachers when you come back
Yes,there is a definite difference. Year 12 is a full academic year and i enjoyed it immensely after the first few weeks of easing into my A level courses. For me the year had a more open ended feel, really of course A levels are not 2 years, they are 20 months. Year 13 is different, courses finished by March then revision, inevitable exams on the horizon and that end of an era feeling,all the normal yearly school events happening for the last time, it seemed to fly by, so many people i have not seen since results day. A big part of your life is coming to an end, if you remained at the same school this has covered 7 of your 18 years. Later on the reality is that you actually only attend secondary school for about 1200 days, in the grand scheme of things its not a lot.