Why would anyone believe the GBSA is a comprehensive list?

The Seattle Academy isn't on the list and literally held the state conference for GLSEN this year ( https://sites.google.com/glsenwa.org/2019-conference/home ).

> Catholic run schools (which are the majority of the private schools in the region)

Going to various directories that let you filter by religious affiliations shows that catholic schools are probably ~20% of the private schools in Kings County and ~15% across Washington State. Christian schools make up another ~25% in Seattle and ~20% statewide. So, while a significant portion, Catholic is definitely not the majority and the combination of Catholic+Christian might approach 50%.

I went to the first two schools alphabetically in NWAIS's directory of members for Seattle and they both had nondiscrimination policies that included sexual orientation and gender identity. Neither is Montessori or language immersion.

Bertschi (admittedly on the GBSA list also): https://www.bertschi.org/jobs

Billings: http://billingsmiddleschool.org/employment

I confess general ignorance of the religious schools in the area, but while this obviously this isn't an exhaustive survey the following examples seem typical of secular schools in the area (all quotes lifted from the school's websites).

Northwest School

"Diversity is our commitment to build [a] community that includes individuals and families who represent a wide variety of ... gender identity and expression, sexual orientation"

Seattle Academy

"includes and embraces a spectrum of differences in ... gender, sexual orientation"

Lakeside

"Lakeside is committed to creating an inclusive and equitable community in which all individuals can participate in and contribute to the life of the school, regardless of ... gender, ... sexual orientation, or any other aspect of their identity."