Should a school representitive attend a mediation meeting, in an appeal for refusal to asses twice

Im finding it hard to understand the process. I am appealing refusal to assess twice for an EHCP. My son yr5.
my son been on the waiting list for assessments for 3 years, so no official diagnosis.
I have sent in the appeal and all the documentation apart from the mediation letter, although had made contact within the time frame. So now appeal has not proceeded until i get that letter or have meeting.
Since - i have spoken with mediation services and agreed to have a mediation meeting with LA.
An appointment has been given. They have asked for a school representative to attend.
I now need to understand . how to respond and proceed, esp if im not happy with the school. Its all very confusing.
It would be great to be able to speak to someone about the whole process and how to proceed.