The case centers around Gavin Grimm -- a transgender HS student who sued the school board in Gloucester County, VA for the right to use the boys' bathroom. In a new legal brief filed with the High Court ... Laverne, Caitlyn and her "I Am Cait" co-stars, Jen Richards and Zackary Drucker ... all sign on in support.
The move comes on the heels of Trump's announcement he plans to lift federal protections on equal access to bathrooms for transgender students.
You'll recall ... Caitlyn lashed out at Trump for walking back his promise to protect the LGBTQ community. The case will be heard March 28. It's unclear if Caitlyn will be sitting in the gallery.