What?? There was almost nothing ecchi about this flash, except for one scene that was canon!! And that was when Harima and Tenma changed into togas since their clothes were wet.
Not quite! It's Sakuranbo Kiss ~Bakuhatsu da mo~n~ by KOTOKO. Or more accurately, さくらんぼキッス ~爆発だも~ん~ by KOTOKO. It's the theme from カラフルキッス ~12コの胸キュン!~ (Colorful Kiss ~12ko no mune KYUN!~).