If people are confused on when and where they did the deed, they had sex on several occasions. First time was at a love hotel, once it was shown they did when they were supposed to be studying, at the ryokan (they probably just played uno with the OLs I doubt it was a foursome) and probably some other times in between. If you look close in chapter 23 there's a red trashcan with tissues in it, they usually use tissues to clean up after sex in manga. (from what I've seen) This chapter confirms that they were seniors in high school, he's 17 and she's 18. Universities usually let students wear their casual clothes not uniforms. She should have been on the pill and he should have used a condom but oops oh well. I agree the author was sonic speed on this and didn't need to go teen pregnancy route, but imo it made this more interesting and different. People shouldn't have flammed him on twitter for his decision. People online are quick to be assholes cuz they aren't face to face, and twitter is one of the worst.