I'm pretty sure she is actually Eighteen-chan because of the brown hair.
Eleven-chan has black hair and two clips on the side.>googles on
After club practice, Eleven-chan pretends to be Eighteen-chan and asks Arata via note to the sports shed, confesses her love and has her way with her in the dark. But Eighteen-chan, looking for Arata, eventually walks in on them in the act (maybe there from the beginning?)...
Drama? Arata/18/11 love triangle/yuri threesome? NTR?
Maybe a writer could figure this one out.