A good instructor will often offer a good class that good students can take advantage of. A bad instructor will never offer a good class and no one can take advantage of it. Avoiding bad teachers is therefore always a move in a good direction, while selecting good teachers is only sometimes a move in a good direction.

I've been working in K-12 and college since 2005 as a sign language interpreter. The qualities of good instructors varies, but the qualities of bad instructors are shared. Late, unprepared, digressive without profit, argumentative, close minded, uses the class as political platform, uses the class as therapy session, uses the class as amateur comedy, unable / unwilling to earn respect (grudging or otherwise) of class - those are some of the things bad instructors share. Avoid those and you're more likely to succeed in any class.