Talk to the person who oversees your school's rules and regulations and ask for permission to leave the notes. Make a list of all the messages you intend to use and where you plan to put them if permitted. This is your draft copy. If you are a student, you might need permission from your student council or the principal. Once you have received permission, you can write out your notes.
Write notes that people can relate to, and decorate them. Research inspiring quotes and words that have been used before for inspiration. Use bright and colorful pens so that they catch people's attention. Include pictures if possible. Use big, bold letters that are easy to read. Be creative. Your inspiring notes can be posters on walls or bulletin boards, messages written on blackboards, or small pieces of paper hidden in obvious places.
Pick the best time to leave the notes -- a time when more people are likely to see them. It could be in the mornings before school, at lunch or between classes. Place the notes in the most visible and yet surprising spots when no one is looking. Distribute the notes over a series of days in various locations so that they circulate around the whole school. Depending on the number of notes, you can also make this a group project that your friends participate in.