Washi Tape is a wonderful teaching tool. Students of all ages enjoy using it. When things get tough during a lesson, I often pull it out to help refocus a student. "Should we put some tape on this passage?" It also gives a little break from working hard and provides a student choice and control over something creative and pleasant.
These brightly colored highlighters from Staedtler were an new addition to the studio last year. They have a great chisel tip which writes both wide and narrow depending on which way you hold it. The kids LOVE them - especially having options like teal and magenta. They used them to decorate their music money envelopes. (Along with washi tape, of course.)
White-out correction tape is a true blessing. I use it to change fingerings, notes, titles and anything else that suits my fancy. This is my current favorite. It works well and stays on the spool better than others.
My students enjoy these Colorful Silicone Cupcake Cases. I didn't know what I'd use them for when I originally bought them, but now I know! They're great for giving a student just a few of anything - Japanese Erasers, stickers - anything where you want a colorful option. They are also good fidget toys as it's nearly impossible to make any noise with them.
Erasable Highlighters are a new addition to my studio. I like to use them when I'm having my students highlight something themselves. That way if they make an error it's easy to change. And yes, they really DO erase!