Alright... firstly, take a deep breath and relax... I mean it, BREATHE!!!
It's really not that big a deal, and even if they aren't interested in talking to you, think of it as their loss and not yours.
Within these kind of situations, the trick is to try and immediately build a rapport with people. You could use some of the surroundings as an ice breaker (e.g. is there a crazy teacher? Some difficult homework? Some sports event? Whatever's trending on Reality TV or Netflix; You could even use Halloween as a conversation starter). Personally, I wouldn't go up and just ask someone if they'd like to be friends, from my experience, most friendship start off as casual encounters and then they grow over time and as you see them more.
Still, having said that, a useful mnemonic when getting to know new people is the FOREplay protocol, I'll try and explain:-
F:- Family:- Ask about their background, do they have brothers and sisters, where are they originally from etc.
O:- Occupation:- What would they like to do as a living, do they have a plan to get there
R:- Recreation:- What hobbies do they have? What do they like to do in their spare time? What TV shows do they like etc.
E:- Education:- What subjects are they doing for their GCSE's DO they want to go to Uni or start earning money as soon as possible?
The trick to do is listen to what they say, and then ask leading questions based on whatever they've responded with. The key here is to come across as pleasantly intrigued without coming across as "nosey" or like you're giving them the Spanish Inquisition.