Let's review. Ako (student) held back a year (or maybe two) for some reason, and it is hinted that she attempted suicide before. Ako (spirit) couldn't remember anything, but when she was asked about her friends, the temperature drops (which I assume as her getting agitated). On the other hand, Bambi wrote a horror short story once during his high school time. Bambi also dropped out of high school for a reason he doesn't want to reveal. Now, there is a plot point about Bambi's "kind and serious" classmate from high school who "must have graduated by now."
Here's what I thought. Ako is Bambi's former classmate, and there was something that involved the two of them. Say, Ako found Bambi's short horror story and liked it (it is shown that she's a fan of ghost stuff, with her pillows and key accessories). Ako then told Bambi how much she liked it and wanted everyone to know about it, but their response was outside her expectation. Maybe Bambi got bullied, or maybe some incidents led to some other, Bambi decided to drop out of school and Ako felt responsible for it. She did it out of goodwill, but ultimately she was the reason Bambi dropped out. And so, she attempted suicide (kinda extreme though, so it's very unlikely). She of course didn't die, but because of her injury/trauma, she was to be hospitalized for some time and she had to hold back a year or two.
Also, Bambi is 18 years old. High School students usually graduate between the age of 17 to 19. Chronologically, it's entirely possible for Bambi's drop out and Ako's attempted suicide happened during their second year. If Ako got held back for 2 years, she would be 18 years old as well.