Nobody can really answer this for sure. It's best to talk to a teacher at your school and see what they advise, i.e. would you be able to cope with the workload and could they structure your timetable to suit you taking lessons. Unless, of course, you wish to teach yourself and just go in to take the exams which is another possibility... Nobody can really say how hard it is or how much work you need to put in though, as surely only you can answer that based on your abilities. As for whether or not there is coursework, that would depend on the exam board you wish to take the exams with, and you can check that on their website online, perhaps? And yeah, it'd probably make sense to do the AS exams in January and the A2 ones in June (usually every module is available at both sessions, but don't quote me on that - check the exam board website again). Good luck if you decide to do it.