7 years old is a bit young to expect a best friend already. At that age it's whoever happens to be on the monkey bars the same time as them every day.
Especially in a public school, where there are 3 classes in each grade, the friends change each year depending on who is in which class. But you mentioned Catholic School, which to me means they probably have one class that sticks together year to year. That can be a little limiting.
I will say one thing, you have to do the inviting at this age. That is pretty much what solidifies a friendship at that age. Playdates, parties, and outings. If you haven't had any girls over to your house to play yet, hosted a bday party, or arranged to meet up with someone at the park or zoo or something, that's probably all you need to do to tip the scales.
I also have a 7 year old girl. She doesn't have a best friend yet but she has quite a few she calls friends. The ones I like, I encourage further with playdates and stuff, and a few she just plays with at school.