This is an amazing organization that creates resources and provides trainings around gender identity and expression. In particular, they have a number of excellent resources for creating gender inclusive classrooms and schools. Some of our favorites from the Gender Spectrum Education page are below. Check them out for even more greatness.
Good for parents, teachers, anyone really. Useful examples of language to use and conversation prompts
This is a good start, but if you want more, check out 12 Easy Steps on the Way to Gender Inclusiveness and Things Anyone Can Do Tomorrow for more ideas and how to work gender inclusiveness into different topics!
This one could be useful to help teachers explain why this is so important to parents who are concerned.
This webinar, put on by Teaching Tolerance & Gender Spectrum, is an accessible overview.
Resources from other sources
This one is really nice because it's quite a comprehensive toolkit with good explanations.
Also from Teaching Tolerance, a good checklist for teachers to think about many types of representation.