God, your straight up just looking for reasons to be offended at this point, clearly more interested in being "right" (which you actually aren't) than you are in actually educating yourself about the topic you're virtue signaling over. The fact that you're entire argument hinges on a specific (and incorrect) interpretation of less than 10 words should be a sign that you're point might not actually be that defensible - like a sandcastle facing against a moderately sized wave.
News flash, school starts in the spring in Japan - in April, to be more specific - not in the fall like in the west. Since the game takes place over the characters' summer break in August, that means that the characters have all been in their senior year for about four months; plenty of time for all the characters to have had their 18th birthdays, which, by the by, can quite often occur during one's senior year or even leading up to it; you're circumstances and experiences are not universal. The characters weren't legal in the OG Morenatsu, but this is not the OG Morenatsu, so it's not fundamentally right (in any way) to judge it as such. Nevermind the fact that this game, in fact, is not actually set in our real world, and that the schooling of a foreign country in a fictional alternate universe populated by anthropomorphic animals - including mythical creatures such as dragons - could very well be different in some aspects compared to your own frame of reference.
Furthermore, a person's level of schooling doesn't invalidate the legal prospects of their age. Would you be making this much of a fuss if the characters were all 40-year-olds who re-enrolled in high school after dropping out when they were younger? No, of course you wouldn't, because that would be beyond ridiculous. If the concept of "suspension of disbelief" is that difficult for you to comprehend, then just don't play; it's literally as simple as that.
If you're that desperate to rip into a game for problematic content, how about going after something like After Class for blatantly admitting that Gil was sexually harassing the MC and other high school students throughout MC's entire high school career? Or maybe the Lagoon Lounge games - both OG and remake - for their veritable laundry list of issues and problematic content, including but not limited to sexual assault, rape, mind control, corporate espionage, and - in the case of the OG games - actual pedophilia involving actual minors?
If you want to be offended by something, that's your prerogative, knock yourself out. But keep your "BuT tHeIr In HiGh ScHoOl So ThEy'Re AcTuAlLy MiNoRs" bs to yourself; no-one here asked for you to spew your SJW preaching in the comments, so get off your soap-box and just be quiet, because nobody is actually listening to you. Just leave your 1-star review and scurry off back to your Tumbr blog or Twitter account or wherever it is you crawled here from.