Seiho Kei

Students are at a hot spring for a school field trip and two friends, including one who happens to crossdress, end up alone together.

A girlfriend decides to dress up her very effeminate boyfriend in a Yukata, but he takes it personal accusing her of loving females only, and escapes to the fire work festival. It's a trap oneshot for gender bender fun, from the same author of The Night of the School Trip.