Hatsumi Sensei made a remark once about how students deserve their bad teachers. This sounds odd at first, until you've met these students or teachers. How do students achieve this?
Not knowing why they want to learn a martial art.
It starts here. This informs all your choices going forward. If you lack this answer or the answer is one you are not willing to share in public, then time to reconsider your hobby.
Not researching the style.
They e-mail after reading my website "Can I ask you what style you teach?" Not only does my website describe our style with a brief history, it takes all of two seconds to type Bujinkan into google or wikipedia. How can you determine if I am a good teacher if you are so uninformed you can't ask intelligent questions?
Not shopping for schools or teachers.
Seems obvious. But many find the one that is: cheapest; closest; fills their empty Monday night slot; has cool trophies; or sells them hard. Spend some time on yourself, shop around.
Not recognizing problem schools or situations.
Too many injuries. The hard sell and having to pay months in advance for classes. The teacher that doesn't teach, his senior students run the classes. Insulting or humiliating atmosphere. Unfocused curriculum. The list is long and usually obvious after a few classes.
Not thinking for themselves.
Examine the style and teacher. Does it make sense? Does it work for you? Are you better for having come to class? Not every student fits with every teacher. Martial arts are very personal pursuits.
Not taking charge of their own learning.
A teacher is simply a guide. You choose how you learn and what you learn. If you are missing something, ask. If something needs clarification, research. If you know you are unskilled in one area, focus your training there. Don't wait for the teacher to read your mind. The class you attend once or twice a week isn't enough. Learning is an attitude.
A teacher that accepts these flaws is a bad teacher.
So bad students and teachers deserve one another.