A few things OP, both as a mum who now has adult kids, who were all anxious about High School, and as an ex form tutor, English teacher and safeguarding lead:
Firstly, it’s High School, not big school. Secondly, the staff, form tutors, etc, have done the transition many times before. Trust them. If her school is anything like the High School I taught at, phones are irrelevant, turned off and left in bags the minute they walk through the gate. Confiscated if used during the school day, so please don’t fixate on this.
Thirdly, don’t compare junior school and what she experienced there with high school: it’s like comparing apples and pears. The stroppy brash kids will be just as nervous believe me, the big fish in the small pond are suddenly the little ones again, with loads of kids, many the size of adults around them. What I’m saying is that they all start as equal.
Finally, hide your nerves. Be positive. It’s going to be ok. She’ll find her crew. Her form tutor will have her back. Let her fly. Don’t pre-empt issues, if they happen, deal with them. It’s a big change, but it happens, so go with it.