Wait...So, you're equating "adult relationships" to the characters having sex? Where and how did you pull that out of what I posted?
Of course high school students, hell, even some middle school students, have sex. That's just what people are going to do. But, you are complaining about the two characters not having sex? If you wanted that, go read the plethora of erotica and pornographic stories on this site and others.
As Dragon32 and Deathpigeon already said, Yvonne and Elsa having sex does not make them adults. YOU were the one that originally posted "adults having adult relationships", which doesn't mean anything.
Hell, I will even go with you about Yvonne and Else not being children, according to the plot of the story, if there were panels that explicitly indicated that either of them had a coming-of-age ceremony that officially signified that they were adults in the setting. But, please, don't insult either of our intelligences by claiming you wanted anything other than sex scenes as your "adult relationships".