Sorry not to be mean but, the first few chapters.. they're doing it at P.E which was outside... how did no other people noticed them naked, etc..
If you're worried about that fact even after reading all the chapters, I pity you a lot. At the athletic competition for stealing the loin cloth, that old guy standing between P.E teacher's thighs is the school principle himself. And I'm guessing the other two holding up the P.E teacher were either teachers or chair-members of the school council. If you find sense amidst these sex-crazed stories your logic will be the only thing which is destroyed. Public orgies and gang rapes are normal in this manga. Please throw away your common sense or you will be shipped to mental hospital by those students.
Omg, this is the BEST!!!