I honestly, deeply hope this manga runs long enough to see the moment on page 2 for real.
I suspect it would take dozens of volumes to get to that point, since the manga has only covered about a year of their middle school days, so far, and they have at least three and a half more years to go until their high school graduation, and by spring break of that year he will be 18, finally, and it's the earliest point they can actually get married.
Though the fact that their high school years will mostly be spent apart due to attending different schools is a concern for continuing the series with the same level of focus on daily life as we've been seeing. At worst, they'd only be able to spend weekends together, maybe afternoons and/or evenings during weekdays.