To be honest, I wish WikiVersity existed when I was in school. The site describes itself as an online "Learning Resource". This particular wiki is a compendium of all the things one learns in school, and is divided by academic level, from pre-school up to the higher grades. If you are currently in school and are struggling with something, be sure to check out WikiVersity, because it explains things that you learn in school in a simple way that can be easily understood.