Ame Nochi Hare - chapters 1-18
Author of this is Bikke, who has a long line of Prince of Tennis doujinshi and it's published in B's Log, home of Togainu no Chi, DRAMAtical Murder manga adaptations... see where this is going? Yeah I thought so too...
Premise sounds kinda dumb, 5 (good looking of course) guys get stuck in school on their first day, lightning strikes and now they turn into girls whenever it rains. Some, as you can see bellow, turn into pretty tomboys, it's all very conflicting... oh and of course the school campus is separated into boys' and girls' side, to make things more awkward.
But so far it actually hasn't steered into any overly BL stuff, heck I could probably say there's more yuri in it if anything. It doesn't really deal much in regards to the physical side of their transformation, just happens, the furthest the story addressed it is when they went to buy underwear, so no Inside Mari scenes (you know which one I mean). The majority of the story deals with relationships forming between various characters and their feelings, sometimes how they see another side to a person when in other form, and it reveals more about characters' backstory as we go along. It's quite a surprise, I didn't expect it to be such a decent read.