Minnesota Public Radio reports that, like any good conspiracy, the trail on how this happened may have gone cold. The hiring coordinator in charge of finding teaching staff resigned over the summer.
It's not clear what about the classes didn't meet their criteria, but it might be that, despite the class description claiming that "you can decide for yourself" whether chemtrails are real, there is not hard evidence that they are real.
Earlier this year, a paper published in Environmental Research Letters asked 77 scientists and geochemists to explore the possibility that chemtrails conspiracy theories have merit. 76 of those scientists said there's no evidence that they have ever encountered anything indicating there is merit to the theory. The 77th scientist was a geochemist who found "high levels of atm[ospheric] barium in a remote area with standard 'low' soil barium," according to Steven Davis, one of the study's authors.