A lot of people like to note that Peter and the Far From Home gang are still in high school even though it was supposed to be 5 years since Infinity War. But think about it.
Couldn’t all of the students in the trailer just be of the one half that disintegrated at the end of Infinity War? If each person actually did have a ½ chance of survival, the fact that one person knows another doesn’t affect the outcome. It’s the same scenario as everybody in Clint’s family but Clint dying; unlikely, but possible, and it totally worked for the plot.
But as to why, or rather how, they are all still in high school…
When Peter returned, he mentioned that Dr. Strange told him it’d been five years since he ‘blacked out.’ Considering that he didn’t look any younger (and didn’t notice anything off about himself), it’s likely that those who died were returned the in the same state as when they left. This would put all the teenagers who disintegrated/de-disintegrated back in high school.
When the snap occurred, schools probably froze their records of the then-absent students. Those who returned would simply re-enroll and continue where they’d left off.
Of course, because this might overcrowd schools, those who return would either a) wait until the following school year to re-enroll, or b) be placed in a special education program (to ‘keep them up to date’ while they're waiting). Truthfully, because the snap affected all ages, the only grades that would feel the bulk of overcrowding are kindergarten/preschool (since there were only five years of possible population growth). Still, students could have been relocated to different schools/schools could have been shut down if a large enough number of teachers or students at that school had been snapped, so overcrowding is a possibility.
However! As a STEM school, Midtown High is more selective and specific and because of this is most likely less impacted by relocation. This could explain why Peter and Ned were both seen at Midtown near the end of Endgame.
Which brings us to Spiderman: Far From Home. It’s senior year, it’s a school trip, and it’s (likely) all the re-enrolled students. My guess is that it’s some trip the decathlon team paid for/Midtown approved of before the events of Infinity War, and now the school’s making good on its promise for those who couldn’t go.
A last thought I want to tack on: one guy I didn’t recognize from the trailer was this dude standing next to Flash:
Imdb listed the character (played by Remy Hii) as Brad Davis, who in the comics was (according to https://marvel.fandom.com/wiki/Brad_Davis_(Earth-616) ) a football player that at one point dated Mary Jane – alluded to in these movies as Michelle). Whether or not that bodes love triangle/romantic snafu, I’m more interested in the ‘football player’ part. My guess is that he’s a classmate (maybe a friend of Flash's) who didn’t end up getting snapped. Given the time frame, Brad Davis could have recently gotten his bachelor’s degree and simply decided to join his (now younger) classmates on their trip.
