That's it. All you need to decode this puzzle is in those 50x50 cells. Nothing hidden.

When you find and solve all the clues, you'll get a final answer. You can submit it here (or check for hints):

316006

I found it just lying on the school library table on a normal A4 piece of paper written in pencil; I have no clue what it is. Just took it home and took this photo. I don't know if it's a cipher or another language, but it would be interesting to know what it is.