I have used CDs as hall passes for several years. This is the second set I made. For the first set, I used an entire rainbow set of Sharpies and drew bubble letters with shadowed lettering and everything. I spent a lot of time coloring on both sides of the CDs and getting a Sharpie headache. By the end of last spring, all the CDs had cracked or broken because kids had started bending them. But, the first set lasted at least 4 years. (I did change out the string just for hygienic reasons.)
Occasionally, while standing in line waiting for their homeroom teacher to come, a kid will notice that the musical notes are not accurate and that the treble clefs only go at the beginning of the staff. I just use it as a teachable moment and have them explain what is wrong and how it should be changed in order to be accurate.