Daughter is struggling with leaving primary school and wants to stay

My daughter has been through an enhanced transition to a mainstream high school, this has went well and she has really enjoyed this. Her last day of primary school is this Wednesday and when I picked her up on Friday she became so upset screaming she wants held back a year and wants to do primary 7 again. She says she is not ready for high school and there are too many different rules in each class. She also became really upset at the prospect of losing her teacher who she has formed a really close bond with over the past year. She is worried she will be forgotten and she is upset that she will never see her again. I feel totally helpless and feel so sad for her that she is losing someone who is really important to her. I am going to speak to the teacher before wednesday and ask if it would be ok to keep in touch via emails for a while. I am now worried sick about Wednesday and how the last day is going to effect her emotionally any advice on how I can best support her would be appreciated.