I am not a teacher.
Mind one's p's and q's refers to:
- to behave properly; to display good manners
- to pay close attention to small details
mind p's and q's - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia.
The letters of p and q should be in lower case, or the idiom wouldn't make sense. I think p and q look quite similar to each other, and people tend to mix them, so people have to pay close attention.