Pupilage?

How hard is it to get a pupilage? I've been hearing a lot of stories about it being difficult.

Short answer - extremely! And for a start, you should know it's 'pupillage'. There were only something like 400 pupillages available nationally this year (2015 start), and number doesn't seem to be changing much. It obviously doesn't mean there are 400 chambers either, as some offer more than one pupillage. I'm sure someone on here will have links to the current stats, or just check the Bar Council and/or Bar Standards Board websites, which should have more info. There is of course also the BSB health warning:
https://www.barstandardsboard.org.uk/media/1363162/final_health_warning_for_bsb_website_24_jan_2012.pdf

So it is doable, but not easy in the least, and then only if you have the requisite academics, experience and extra-curriculars. What stage are you currently at? I'm guessing school?

Sorry for the typos (currently on my phone) I am in A2 with a AAC (resisting one module to move the C up). I'm going to be applying to Kings College and LSE as my grades aren't good enough for Oxbridge.

What extra curriculum activities are good for students wanting to go into a career in law?

If you can get into KCL/LSE, that is a good start. Are you going to be studying for the LLB or another non-law degree?

If you're aiming for the Bar, you'll need to get some mini-pupillages under your belt, most of which can be applied for once you've started your LLB, though some will require that you're in the penultimate year before you apply. If you're doing a non-law degree, some sets might let you do minis during the degree, though most will require you to have started the GDL.

For the solicitor's profession (which is easier to get into than the Bar), you will need to do vacation schemes, most of which I think you can apply for from the 1st year of uni.

In either case, it would also be good to have some experience of the other profession, so that you can better justify your choice (eg. if the Bar, having one VS in addition to minis, so that you can explain in applications why you chose the Bar over being a solicitor).

You will also need plenty of extra-curriculars like mooting (usually at uni), debating (might also be available in your school), other forms of public speaking, teaching, etc., as well as things like work experience (paid jobs, preferably in an office rather than retail/waiting type work) to show you can handle pressure and are organised, and voluntary work. You will also need a way to show commercial awareness, especially if going for the sols profession, etc. Obviously, this is not an exhaustive list and not all of this will be 100% necessary for both professions, so you will need to do your homework once you get to the appropriate stage.

In either case, you can get most of the exp you need at uni, though you will need to be proactive in seeking it out (eg. trying to get onto mooting teams), rather than waiting for the opportunities to come about on their own. Having said that, the fact that you're asking these questions now is a very good start.