In grades 1st to 3rd, I recommend saying the Our Father at the end of faith formation classes. Displaying this free printable poster of the Our Father helps children who have not memorized the prayer yet. (See this post for posters of the Hail Mary).
These 3 different sizes should meet your classroom needs. The letter-sized one can be mounted on the inside of a file folder for easy storage.
Print a size that works for your needs.