So tl;dr my school has a wall we're allowed to paint whenever we want and right now it's full of really shitty stupid graffiti instead of some art and I want to try to change that.
So what would be the best method of adding something to the wall that will stay until someone paints over it?
The only thing i can think of is stencils and spray paint but i'd rather just apply directly to the wall and go nuts.
But i don't really know anything about outdoor paint.
Here's a world's wealth of knowledge
http://lmgtfy.com/?q=what+paint+to+use+for+outdoor+murals
You can use acrylic paint as long as it's a reasonably smooth surface. If it's very rough fluid acrylics may work, but stencils and spray paint or acrylic markers are probably the cheapest and easiest choice.