M.R.
I haven't read the other responses so I don't know if I'm repeating. If I am repeating though, it might be nice to hear it again.
I was an introverted kid. Not shy. Introverted. I had 3-4 close friends all through school and was fine with it. I was involved in sports although none of my friends played nor did I become really close with any of the girls on the team. I was happy then, I was happy in college, and I'm happy now.
I don't have boatloads of friends and I'm okay with that. I've never really been the social butterfly type and prefer a handful of really strong, close friendships. I don't feel like I missed out on anything but, quite frankly, I don't think I would have enjoyed always being on the go, socially.
If she seems happy and the teacher isn't concerned, then I wouldn't worry. Not every kid is an extrovert and that's okay :)