My DD skipped K and she was 10 when she started MS. So, she had all her friends going to the same school and everything was smooth. One of her best friends is at least MG and probably PG and I reached out to her parents and both of them got enrolled in Algebra 1 class. She has at least a friend or two in each class.

But in middle school, the kids starting to group into cool kids crowd, nerd crowd, etc... and it could be bad especially for girls (more so when they get into puberty and teenager years). If they don't aleady have hobbies, make sure to keep them busy in those years.