School colors make a natural theme when decorating the doors to a Catholic school classroom. You can use posterboard in your school's colors and make signs to hang from the doors. Or, use crepe paper in the school's colors and hang them like streamers around the edge of each door in a hallway.
If your Catholic school has a mascot you can use that mascot to make a charming motif for doors, particularly if the mascot has a religious theme, like the Angels or the Saints. Use stencils to paint the school mascot onto doors using acrylic paint. Or, draw out the name of the school mascot across doors use paint markers and letter stencils.
You can choose a few favorite psalms and write them above the doors to the classrooms, using permanent marker. Make sure to add where in the Bible each psalm is located. You can also use Bible passages in this way. Get students involved in the activity, particularly those who have nice handwriting.
Get art students with a talent for painting involved in the door decorating process. Students can get together and paint scenes from favorite Bible stories over classroom doors, making a mural. Each door can even tell one scene of a complete story, progressing as you walk down the hallway.