Bad Fairies is a class that Mal took in her first semester at Auradon Prep. Little is known about this class because it is mentioned in Mal's Diary when Mal wrote her schedule. It is most likely an elective class since at the beginning of Descendants, Doug lists the course requirements and it is not one of the classes mentioned.
Descendants: Mal's Diary
In Mal's Diary she lists her schedule (for her first semester) it is
History of Woodsmen and Pirates
Enchanted Forestry
Bad Fairies
Mathematics
Lunch/Recess
Grammar
History of Auradon
Safety Rules for the Internet
Remedial Goodness 101