She will adjust.
This is how schools are.
They do not, keep all the friends in the same classes year after year, so that all the kids in each grade level, will get to know each other and other kids... and then once in middle school, they will look out for each other and help each other.... because they have known each other, per being in classes with them.
There is a reason for it. That is the reason, that my Daughter's school/Teachers have explained.
One year, in 1st grade, my daughter did not have ANY friends in her class. Plus, that class was full of obnoxious kids. My daughter, was not bothered by it at all. She even told me... she didn't care because she does her own thing and plays with her friends at recess. She did not want to make friends, with those obnoxious kids, anyway. She was totally, self-reliant about it. She enjoyed her class and the year, nonetheless.
My Daughter entered 1st Grade at 5 years old, and then turned 6 in 1st grade.
My Daughter, was Shy too.
She was fine.
Because she is also, self-assured. Shy... does not mean, insecure. Those are 2 different things.
I was proud of my daughter.
She handled it well.
Kids adjust. Even if their friends are not in class with them year after year.
They also make other friends too, and increase their social aptitude. Of which you guide them on it.