Very interesting manga. I can say with pretty certainty that the school part happened in the past.
Haruki's chapter (ch 9) put's him awakening from the coma to around 2035, at around 12 YO. Him being 18 would put this 6 years later, or happening in 2041. Because we know the cataclysm happened 15 years ago when he was 3, we can see it happened in 2026.
In ch 17 there is an timestamp on one of the cameras that reads f17/10/28, and Japanese date format is YYYY/MM/DD, pointing to the happenings being in 2017 or 2028 if the year is last. Neither of the dates really match when the disaster should have happened tough, so that might have just been Aperture/MM/DD , even if an f17 aperture is really weird for that context.
Some way it all could make sense is if 2035 was the current date, which would set the disaster for 2020, which would match really nicely with an 2017 date on the recordings.