Chris' vision of the reunion:
Everyone that he knew shows up, it's held at the MHS auditorium, and he would have a chance to re-do his prom & graduation experiences, except this time it would all go right. He would dance with Tiffany and his gal-pals, and would get some awards praising his creativity and recognizing him as the man who made the reunion happen.
Final goal: Re-live high school and reconnect with his gal-pals so he can choose one to make into his sweetheart from the ground-up.
How it would really happen:
Assuming there is any interest at all, hardly anyone shows up. Most people have moved on with their lives. If left to Chris it is held in some public place where people can just show up, if he has help it might happen at some rented meeting room. Those who do show up are locals who had nothing better to do that night, and almost certainly some "trolls" will be present. Chris will have been aware the trolls know his reunion plans and be paranoid of everyone and will be checking everyone's names in his high school yearbook (although that won't keep trolls from assuming one of those identities). Maybe a Man-in-Pickle-Suit shows up, Chris would be outraged and terrified, while his classmates find the M.i.P.S. amusing and welcome him as the highlight of the otherwise dull event. Although Chris would anxiously be awaiting their arrival all night, none of his old gal-pals will show up. If any women do show up they are with their husbands. No one will want to dance with Chris, who despite making a half-assed effort to groom himself for this special night will look 2 years worse than he does today. Chris would get a feeble round of applause for his part in organizing the pathetic attempt at a reunion from the few people who don't leave early as his only reward for his efforts. Based on how shitty the 15 year reunion would be, his classmates will never have any interest in attending any future reunions, killing any chance that a 20 year reunion would ever happen.
Final results:

(and

if the Pickleman shows up)