"Boy's Kingdom" by Aya Oda 1 volume -complete
Himeno transfers to a school formerly exclusive for boys. She has to deal the unruly boys and the Hallelujah Student Council. The heroine looks ordinary and the president looks weird with his undone shirt buttons. I find the manga serious. If you want a light and funny story with a girl classmate in all-boy school, try "Rockin Heaven'" or "Dear School Gang Leader". This oneshot has a prequel which I haven't read yet.