Erin,
The fact that you were involuntarily transferred must make this situation extra frustrating for you. Also, since this happened in October, I am assuming that your students were removed from the classes in which they began the year and moved to your class, not a comfortable situation for them either. It is likely that the parents whose children were moved did not volunteer for the change either. So you are all in unfamiliar and challenging territory, which probably increases everyone's anxiety and impatience.
A plus in this situation is that you are knowledgeable about and experienced with the kindergarten curriculum and expectations. This provides you with skills and insights that many first grade teachers do not have, which can be a distinct advantage when working with students who are struggling.
The student who arrived in November and has a behavior management plan raises some questions for me. Is his plan part of anIEP or 504 plan? Did you design it alone or in collaboration with special education personnel and his parents? Part of the purpose of a behavior management plan is to allow the child to function better without requiring the majority of your time. If this plan is meant to prevent him from harming himself and others, and it is not working effectively, it needs to be revised. If he is not able to be in the classroom safely while you teach, arrangements need to be made that will allow that to happen. For example, the plan could include removal from the room to a safe cool down place such as the principal's office, a counselor's office, another classroom (assuming he doesn't act out with older children, etc.) This child might also need a one-on-one associate. I recommend that you advocate STRONGLY for this student's welfare and the welfare of your other students by insisting that the plan be workable and allow everyone to learn. I know this can be a long and frustrating process, but I also know that if you just tolerate it, the child will move from year to year, classroom to classroom without appropriate services.
It is very frustrating to feels like you are babysitting and teaching manners more than content, but teaching behavior absolutely has to be a priority in your situation. Without it, you can't hope to make academic progress. If you give yourself permission to make that your focus, you will eventually be able to teach content the majority of the time. Until your students are able to pay attention and get along, you will need to spend class time building those skills. These "academic behaviors" must be learned, which means they must be purposefully taught rather than just responded to incidentally.
I also wonder if the parents of the students who are behaving well have voiced their concerns to the principal. In my experience, parents carry more weight in these matters than the teacher, and help tends to come more quickly when they speak up. I think it could be helpful to solicit their presence in the classroom as helpers, visitors, etc. so they have a clearer picture of what is actually going on. When they witness the challenges you are facing, perhaps they will be motivated to either help more or advocate for additional help in your classroom.
I hope at least some of this helps. In these days of high stakes testing and increasing academic requirements, it is becoming more and more difficult, in my opinion, to focus on the children rather than just the academics. Unfortunately, when people become secondary to subject matter, our students do not learn how to learn, to cooperate, to accept and respond to redirection, etc. and their academic progress suffers as a result. I hope your system will allow you to address the needs of your students so you will be able to help them grow academically.