The Daydream Theory
Ricky seems CONSTANTLY spaced out in this movie. The part where he is daydreaming about Joanne and Deadgirl is clear. But shortly before this, it cuts from the basement to the school and he's absorbed in drawing a hanged ghoul during class. This theme is repeated again while he's sitting alone on the bleachers zoned out, and Joanne comes up chasing a basketball. It also cuts from the basement to him throwing a tantrum, but I guess that could go either way. Depends if he has blackouts... but he is prone to fainting!! The fainting could signify breaks in the dreaming at points that shock him awake.
None of this is evidence, only foreshadowing. The real evidence is at the VERY beginning, and the shot right after Ricky is holding Joanne and she's stabbed. The shot I'm referring to is of Deadgirl running through the brush. It has the same slow fade-in effect as the very beginning. It then cuts to Ricky DAYDREAMING in class!! He has a completely different appearance and carries himself differently, posing the question is Ricky just an alter ego? Open and shut for those who choose to interpret it this way.
The guy in the ending might not be Ricky anymore, but a guy everyone knows as Richard. He is shown sitting at the foot of a tree with a comic, appearing to have no friends. A guy is seen romancing a girl up against the fence in the same way as Joanne and her jock boyfriend, but it's not them. A random dog is shown, but it's not the same random dog from the basement. The ending is suggesting that Ricky is a normal guy on the outset, but is tormented by his social failings and finds comfort in demented fantasies.