If you are planning on adding an artistic touch to your walls to make your room more inviting and playful for your students, I recommend Drawing out the Design before you commit.
You can use a fancy app like Procreate, or simply use the markup feature available when you click edit on the picture from your camera roll.
As for carrying out the design, not many schools allow you to paint on the walls. If your school doesn't allow painting on walls, use butcher paper or colorful fabric.
Here's what I recommend if you're working with a big design like this sun or rainbow I did in these classrooms (I wouldn't recommend this for smaller designs):
1. Staple butcher paper on the wall. It's easier to draw the design directly on the wall versus somewhere else and then transferring.
2. Draw your design. If you feel comfortable free handing your design, go for it! If you don't, project your image onto the paper.
3. Color your design. I use sample paints from Lowes and Home Depot, but you can also just buy acrylic paint from any art supply store if you're interested in painting instead of coloring. I recommend doubling up the butcher paper so it wont leak through onto the walls. If your school has colored butcher paper and you only need a solid color, like I did with my sun design, then go with the colored paper instead.
4. Cut out your Design. When you are done with your design, take an x-acto knife and cut out any excess paper.
