This depends on who you are planning on inviting. Most schools will do a "dinner" in her honor with current faculty and administration invited- usually a dinner for all of those retiring in the school or district. Check and see what is already planned.
If possible, I would suggest having something with your family and friends b/c they won't be included in the "school celebration". You could reach-out to former students and parents and either invite them to attend or invite them to send letters and pictures and do a scrapbook for her.
Think about the shear number of people she has worked with in her tenure and the things she has "seen and done" as well as the things she couldn't do b/c of her career. Teachers simply can't go on vacation in January... maybe have your family members contribute to a great vacation she always wanted to take but couldn't.
As an educator I can tell you that you won't know which children and memories are the most prominant- only she will. There are some kids that touch your heart and never leave- there are some parents who do the same. I would suggest focusing in on trends, decades and overall celebration of her commitment to a very challenging and (unfortunately) under-valued profession.
Congratulations to your mother!