How to get rid of this would-be stalker? READ DESCRIPTION PLZ?

Ok, so me and my friend are sitting on a bench.we see a girl ,lets call her jess, and we walk past on our way to the bin and say hi.we keep walking.We turn around on our way back,and she is right behind us.we say hi again,and ask her where her friend is,she replies, inside doing her homework.So we offer to hang with her for a while.It's now maths.At lunch,we go to our usual ench and she is following us.We ask her politely if she could leave ,as we need to talk about my friend's (lets cal l her kayleigh) boyfriend issue and its kinda private.She replies , ok.we start chatting.she comes back and says ,oh my friend is inside again.But we can see her BFF sitting on a bench(lets call her alexandra) and we say no she isnt,and then she says ' oh...yeah...bye' and walks off.Ten mins l8r, she arrives,with alexandra ,stating ' we are just sitting in this bench which is now our usual spot.bye.' and then we go. she automatically put them in our group project,excluding my other mate, lets call her georgia. So,she just grabs kayls water bottle and starts drinking from it, and when asked why she says ; UM,since we are bff's we share everything now ! ... at last break she wouldnt go away even when she got in trouble.alexandra is excactly the same.But the wierd thing is,she kept taking pitures of us all and sending them to her ' mates' . And me and kayleigh are the only other ones apart from alexandra to get a Bday party invite.How do we get rid of her ? BTW we are under 14