They probably realized that you're doing just fine and don't need an inspirational message. Or they hate your guts :D

It might of been the most inspirational or best sounding note, that someone wanted to be a prick and took yours instead to feel better about themselves. Don't feel down about it OP.

Maybe it fell or something. Sticky notes aren't that strong. But I'm sorry OP):

I bet the sticky note just fell off, OP! Don't think too much about it, I'm sure the mysterious writer holds no negative sentiments toward you.

Maybe it fell off, or somebody else took it off for some reason? Or they ended up being short a sticky note (even if ones on either side had something it could have just been like 2 people did it and met in the middle kind of thing, so yours got missed)... maybe even they were short a few (unless it's a really really small school I doubt you were able to notice whether every locker had one or not). I wouldn't worry about it, people who go out of their way to do something like that likely aren't trying to exclude anybody and genuinely want to make everybody happy, it was probably just a mistake or bad luck of the note falling off or something. Just know that you're awesome and a note on your locker isn't necessary for that to be true.

Everyday of your life is a page in your history. Why not make a book worth reading. There you go OP; you've now got one too :)

Look on the bright side, whether it fell off or not (which it probably did) - I bet a lot of people thought you were the one leaving all the nice messages since there wasn't one on yours!

Don't worry, most 'inspirational' messages are generally just a crap waste of time anyway, with nothing useful to say...

Let's go with someone stole your note or they went in a specific order and get ran out...yeah that sounds better...

Some people need inspiration from others to perform. There are other self motivated people that do their best without needing any help.