Here's a Peanuts cartoon that I don't think has ever aired on television.
Instead, It's Dental Flossophy, Charlie Brown was shown in schools as a teaching tool for elementary school students, produced in 1975. Here, Lucy schools Charlie---as well as an eavesdropping Snoopy & Woodstock---on dental hygeine.
Unfortunately, Snoopy couldn't leave well enough alone and stole Lucy's floss to create a nest for Woodstock. He always seemed to do Lucy wrong no matter what.