I was wondering if a school can host a female non-binary and transgender field trip or event and not include males. This is a public high school in California.
This page sums up the state of federal law regarding sex segregation and schools. There are various "separate but equal" provisions under the law, for example Boys State or Girls State are not outlawed, there is no requirement for co-ed PE class. But as far as I can see, a school field trip to hear the Petaluma Symphony Orchestra could not be limited to only females (or only males). It's not clear what you mean by "host", or whether the event is educational (the law doesn't say that everything a school does has to be sex-neutral, it prohibits "discrimination under any education program or activity receiving Federal financial assistance").