The sense of confusion you got after Kim Chi woke up at school and found out everyone is still alive...

SNAP BACK TO REALITY when Na Ra woke him up from the shooting scence followed by a fore event of Jang Soo getting attacked by a sphere then turned out to be Yeong Soo starting off a mascare,
a part of me wished this wasn't true and would be saved by anthor SNAP BACK TO REALITY moment but it didn't...

The ending rapping scene got me smiling while wiping my tears like (those mfs have just broke my heart how could they sing just like that)

My takeout from the ending/show is when Kim Chi said "The most important thing in life was the warmth of my friends"

The ending is very witty and depicts the delusions and scenarios we make up in our minds to overcome the pain

the show made me feel every humane feeling deeply and that's enough for me

Ohh... and the very ending scene of their graduation really twitched my heart, I don't see it as an alternative ending
it's a consolidation from a parallel universe