Miss Lenhart from comic 499: Scantron, the first named appearance.
|First appearance||135: Substitute (mentioned only)|
Miss Lenhart is the go-to character when Randall needs a teacher. She has white hair of shoulder length, and is thus a named version of Blondie, like Rob is a named version of Cueball. She may or may not represent the same character from comic to comic.
Miss Lenhart has only been named two times where she actually appears. The first time she is both drawn and mentioned by name is in 499: Scantron and the only other time (so far by comic 1731) is in 1050: Forgot Algebra. But her name was used before for a not shown teacher in 135: Substitute, which is why it makes sense to call the math teacher in 263: Certainty Miss Lenhart. That would thus make it the first time she appeared as a character in a comic.
Her family name is again the only thing used in 416: Zealous Autoconfig, although there it does not directly say she is a teacher. While school is mentioned, it is for her children. Thus the name is first connected directly to the blonde teacher in her fifth "appearance" after she was also drawn in 442: xkcd Loves the Discovery Channel.
Later her name is used again without her being present, except on the phone in 704: Principle of Explosion. Here, she is Mrs. Lenhart. It is possible this character is the mother of Miss Lenhart. Or, because she here obviously has a son, she may have been married after she was originally named miss. This is the second time she has been mentioned to be a mother. Both Blondie and one of the other named Blondie-like characters Mrs. Roberts are shown to be mothers.
Her name is mentioned in total five times. So the other comics with Miss Lenhart refers to a teacher with her appearance, just like Megan only has been named a few times, but is still used to describe the general dark haired woman. However, Miss Lenhart must have long blonde hair, so a teacher with her hair in a ponytail, like in 982: Set Theory, is not Miss Lenhart but Ponytail, even though the teaching method reminds of the one in 1724: Proofs with Miss Lenhart.
 Math teacher
There are several cases where Miss Lenhart is described as a math teacher. Her name is used in this context in her first comic 135: Substitute and she is both named and drawn in 1050: Forgot Algebra as a math teacher. She is also teaching mathematics in 263: Certainty, 622: Haiku Proof and 1724: Proofs.
But mainly it seems she is teaching undergraduate classes. It is therefore not clear if any mathematician looking like Blondie is also a representation of Miss Lenhart. But if that were the case these comics may also reference Miss Lenhart:
But so far they have not been included as Miss Lenhart as Blondie is not clearly depicted as teachers (or with name) in these comics.