Miss Lenhart in comic 499: Scantron.
|First appearance||135: Substitute|
Miss Lenhart is the go-to character when Randall needs a teacher. She has white hair of shoulder length.
Miss Lenhart is usually named when she appears; however Randall uses her visual appearance from time to time, making it open for interpretation whether it's Miss Lenhart or not. The aimless student in 59: Graduation could for example be a young Miss Lenhart.
At 704: Principle of Explosion she is Mrs. Lenhart, but maybe this character is just the mom of Miss Lenhart.