I'm wondering if anyone knows the title of a rather old ecchi manga which I think was printed sometime before the year 2000. I remember it being about high school students and there is one chapter where the two female characters visited a weight-loss centre or spa in which the male protagonists disguised themselves as the staff there so that they can do perverted things on the female characters including performing fire-cupping on one of the girl's breasts.

Nevermind. I found it. It's Yarukkya Kishi.