In the real world, yes, 18 is the age most American teens will be when they finish high school, but this is the BaDIK world where the rules are often different. But even in the real world, not every country is the same because it's quite common in many European countries for the high school graduation age to be 18 or 19 because they spend longer in that education system, so perhaps this is another amalgamation of the American and Swedish systems as DPC has said before that this game is sometimes a mix of the two.
Also, Maya's Dad could have been keeping Maya in the house throughout the summer and therefore keeping her from seeing Josy.
Their relationship was a secretive one, not least of all because of Maya's Dad, so it's difficult to build a really strong relationship when they're so much uncertainty and insecurity surrounding it. We also don't know when Maya's Dad found out, which is something that will hopefully be known at some point.
People assume they've only been apart for the summer, but what if he found out weeks or months before then and pulled Maya out of school? He could have told the school she was unwell and had the school provide material for her to be homeschooled which would still allow her to graduate from that school (as per the picture) because she was still considered one of their students.
Then during the summer he kept her in the home and, from what we know from the game, they were still able to communicate but it would often lead to disagreements and fights between them (Maya makes this clear in Ep 3 when talking to Derek). So really, their relationship was always on shaky ground and it's easy to see how it didn't withstand this pressure from Maya's Dad, and even after Maya got to college, Josy may not have felt like she wanted to see Maya and so that's why Maya didn't hop on a train then to go see her as she sensed this about Josy.
Big difference with AL is that the throuple hadn't started yet, whereas we can be properly in one in this game. In AL, the MC could salvage only one relationship out of 3 potential relationships, here in this game we have 4 potential relationships when you consider M&J as counting as 1, so if this game follows the same logic as AL, then M&J is a relationship that can and should be achievable.
If M&J aren't also with MC, then it probably is doomed to fail because he seems to be an important pillar in the throuple, like a missing piece of the puzzle, which is why the throuple seems like it might have a better shot at working out.