Hi mman,
Say something like "do you wanna go watch a movie or something after school? we could bring our mates along if you want...." that way she knows it's her that you are after, but you are happy to have her mates, and yours there if she would feel better that way.
Yes you could ask via online messages, but personally I think that's the wrong way, it may be harder to ask in person, but it feels so much better when you are asked in person, and don't get anyone else to ask, do it yourself she will get just the best feeling from it.
Don't leave it too long before you get the gutts to ask, I know it's hard but I learnt the hard way.