Hey man! Coincidentally I just started my second year of sixth form (A2's) and also did not have the best last two years of high school, so I can relate quite a bit. Advice that I can give you is to NEVER let your stammer define who you are as a person and never let it control or take over your life. You want to make new friends? Just walk right up to people and try not to think of your stammer and more so focus on what your saying than how your saying it. If someone laughs or grins if you stammer, they're not worth your time and you should just walk away right there and then. This will give you a 'I don't care what you say' attitude, which will eventually build your inner strength and confidence. Last year when I started sixth form, I moved from a different school, so everything was new and unfamiliar, yes, my speech was not the best and I wish i had the mindset in which i have established now, a year later. Do not make the mistakes that I made...Be outgoing and just talk, it really helps...trust me, you'll be surprised and hey if your able to make new friends out of the experience, you've got a result! Keep fighting the good fight, friend.