One of my DDs started school for the first time at age 10 (having been homeschooled before) and it was a bit of a transition, but she did great with it after a couple of months. The teacher paired her with a "buddy" to show her around and kinda teach her how school works.
I think if you show confidence in your DD that she is a strong person and can do this, it will go better for her. Sometimes as parents we tend to think that if our children go through something difficult for them it will harm them. But a challenge that a child can met successful and overcome helps them build their strength and self confidence.
My advice would be to encourage her to get involved in an activity like girl scouts *or whatever would be fun for her* and really listen when she talks about her feelings. But have faith in her that she can make new friends.
Because she can.
but everything has pros and cons
My two changed schools due to a move at 8 & 10. It helped that we moved into a neighborhood where there were kids their ages at the start of summer. They had 2 1/2-3 mos to make some friends, so walking into a new school wasn't so daunting. Both also had really good teachers who made sure to foster interaction between everyone in class, so they got to meet a fair number of kids in short order.
I also signed them both up for Cub Scouts & Brownies. Regardless what you may think of them, it was a great opportunity for them to meet another subset of kids, who they could do stuff with.
We've used the same tactics as pp - buddies from the class etc. The suggestions for activities outside of school, to provide another group of friends, are very good too.
It's nice if you can find a kid or 2 in the neighbourhood before school starts, so there are a couple of familiar faces. If you don't seem to be meeting anyone, try the neighbourhood library or community centre. There are often summer programs running on a drop-in basis.
We switched schools with one child in grade 3, one in grade 2 and one in primary. Our oldest had lots of issues around socializing, but actually he ended up doing better after the move. It gave him a clean slate with teachers and friends. The family periodically visited the house and the community on weekends for a couple of months before moving (I realize this is a luxury not everyone can have) and we had a week solid to settle in before going to class. We made sure all three (but especially the one who had a harder time with making friends) had some extra curricular activities set up and we just really encouraged everyone that they could make things work out.
Busy keeping up with three children and an awful lot of chickens!