I read somewhere that if you need a co-teacher to help translate and run the class then you aren't a good teacher.  What are your thoughts?

I personally am of two minds on the subject, and I would like to see what others have to say. Like maybe for new teachers this is okay, but 10 plus years still needing someone to be by your side might be a little extreme.

People can get by without a coT, but it's better to have someone who can speak the language and who has the authority to enforce higher levels of discipline. Students tend to be easier to control when you have two teachers getting on their backs, especially when one teacher is a NET who can't speak the local language (students will test NETs more than they will KTs because of the language barrier - they try to take advantage of that).

In many cases, whether or not you actually need a coT is moot. In public schools, for example, it's required by law for liability reasons.

I'm in the camp of...having a class of beginners being taught by a foreign teacher who can't speak the local language without a coteacher in the room is a recipe for disaster.  Apparently this happens a lot?  I can't wrap my head around how any learning can take place in this situation, let alone disciplining.

In many cases, whether or not you actually need a coT is moot. In public schools, for example, it's required by law for liability reasons.

I'm sure you're right but I've still never seen this documented anywhere.

I do remember reading about a native teacher who would wait outside the classroom door until the coteacher arrived, even if the students were inside.  Straight out refused to go in without a CT lol 

never had a coteacher, don't know what they're for or what I'm supposed to do with them..
4 years in and don't speak Korean. It might be nice to be able to use korean sometimes but other than that?

I never had a co-teacher at my high school, which was pretty sweet. The biggest problem I had was students sleeping in class. I couldn't imagine life without co-teachers at my current school though (big public middle school). Things are chaotic enough with them in the classroom.

I read somewhere that if you need a co-teacher to help translate and run the class then you aren't a good teacher.  What are your thoughts?

I personally am of two minds on the subject, and I would like to see what others have to say. Like maybe for new teachers this is okay, but 10 plus years still needing someone to be by your side might be a little extreme.

no. waste of time. having said that, i get paid by the class at my school, and i get paid for sitting in on ("co-teaching") the korean teacher's lessons. don't know why... there are only 10 kids per class :/ not that i'm complaining

Could you guys elaborate a bit more on the behaviour of your students? What exactly makes it difficult to teach Korean middle school students? I had always believed them to be very quiet and well-mannered?

I read through all of the comments and can see where everyone is coming from even though I have only taught elementary. I will just address the OP question.

Yes.
First of all it is in contracts that during regular school hours that a KT is in the room( as mentioned). I personally have a wide variety because I teach with all HR teachers but have really well behaved students (mostly). Those class hours are also part of their weekly hours so they should be doing their part. It could be just helping low lvl students or only working on behaviour or even just being close. In hagwons there is usually cctv, but what about public? There is nothing.
How do you protect yourself and the students in case of an emergency? How do you solve a student dispute/crying (even with really good Korean skills?). Who will back you up if a parent complains? I will tell you right now you will be thrown under the bus. Never happened to me but I know people who it has. Korean teachers/VP/P have a lot more riding on making them self look good. Have you ever seen a Korean teachers open class? Is it anything that you have ever seen day to day?

I guess I asked more questions than I actually answered. I always let the really low English lvl teachers do their stuff outside of the class but make sure that they are close enough that if there is a problem where a student needs help they are there. If teachers tell me they need to talk to the VP or something during my class that is fine but at least I know.

TLDR: Do I need a CT? No. Should there be one? Yes

Language learning is facilitated by social context. People learn much faster by not understanding you than through listening to translated sentences from the Korean teacher.

linguistics-CREED constructions-rational-exemplar-emergent-dialectic (Ellis, 2006)

Explicit Learning in the L2 Classroom: A Student-Centered Approach

Ronald P. Leow - 2015 - ‎Language Arts & Disciplines
... “associative-cognitive” CREED, in which each letter represents the mechanisms of Construction-based, Rational, Exemplar-driven, Emergent, and Dialectic.

There has been an interesting study done on which grammar constructions Koreans learnt in a natural L2 environment. They first recognised -ing forms, then past regulars and very late in the process 'give something to him or her'. The Teachability Hypothesis claims that actual learning only takes place when a student has made a construction part of their interlanguage, not when we force them to fill in grammar tests.

I think experienced teachers will recognise that as well, this is basically how you teach kinder/elem. So.. at first yeah if the learner has 0 level, a lot of showing (context) is involved. But the EDP forces the best communication and often that is NOT learnt from us teachers, but from other social groups (friends or TV). The social activity (Piaget) in the NET class induces these constructions to be recognised and then the grammatical understanding of the language 'emerges' from its usage. I had the same but then with Korean, studying Korean grammar and words at a language school was useless, by watching TV and using it with friends I started to 'understand' the language. I see many wonderful teachers here creating fantastically well devised products which they unknowingly create following these scientific principles. (Ellis, 2006; O'Grady, 1995)

Edited to answer your question

how social context is provided in the classroom

A student wants to complete a social task for which he needs to ask information from an 'expert'. (Piaget) Learning a language is therefore not a grammatical endeavour but a social interaction in which language is a tool to extract the necessary information from the social group that person is in to complete a task:

 Which items will you bring with you to a desert island? Make a list and describe them.  Teacher, what is a jerrycan?

  If you as a teacher give effective feedback (do not correct mistakes but give the student prompts: He not have!  Do you mean...He ...d.. He don havu! Well, he...? Then the student learns English from an expert (can also be classmates) because he has to produce a correct message (dialectic) to fulfil a task/activity. This is why your Korean co-T should never translate nor should we 'recast the correct sentence' when we correct students. Just indicate that the utterance was wrong and help them very gradually to the correct answer. ( I think that's Lyster& Ranta, 1998?).

 The better you design your tasks/activities the more learning takes place. https://tefltastic.wordpress.com/
Now, you may have seen me lament a video of a hagwon which had a really great linguistically sound description of their programme, but when I saw the teacher show the programme on Youtube and drill the students I was ROFL at how bad it was. The activity should free you up as a teacher so you can become an expert who walks around and answers questions about the project. The actual language learning takes place while students are doing game/writing tasks or anything else independently without us.
You could facilitate or enhance this learning process in their ZPD by planning ahead and using the grammar constructions purposefully, months ahead when they appear as 'study material'' in a unit. I am actually planning on doing that when I'm starting the new year.

I could actually claim that Korean students learn much more from the NET than from the grammar classes with Ms Pak/Kim/Jung provided Ms Pak isn't translating each utterance into Korean.