You know, one thing I thought would've made Hiro's title make sense, which they never touched on again, was that his "flunking 3 times" was actually in reference to their predicament.
Like he subconsciously realized that 3 years had passed, but couldn't make the connection, so just thought it'd happened in the past.
No, hiro is actually in his 20's. He flunked out of high school 3 times before getting to Hope's Peak.