Give your Korean middle school students a pretest. The pretest should include both a written evaluation of their English writing and a speaking evaluation of how well they can speak. The written pretest can have any type of questions, such as their name, their favorite past time and their family. The point of the written test is not to see what specific language pieces they know but to see how well they can express themselves in writing. Then sit down with each student for a spoken pretest. Ask them questions that require them to answer in the present, the past and the future tenses. See how well they preform.
Make a list of your students and give them a general rating based on how well they can write English and how well they can speak English. Even if you have to keep all of them in the same class, having an idea of which students are at which level can help you as you teach your entire class. You will also know from your tests what issues you need to teach. For instance, if no students can speak in the future tense, that will be a major issue to cover.
Create an instruction series based on the needs of your students. From your evaluations, you'll be able to have a list of things that must be covered, such as accents, spelling, past or future tenses and vocabulary. Decide on a list of things that must be covered first.
Design lessons based around the list of things your students still have to learn. Lessons should be immersion-style, meaning that you will speak only English to the students. Students will learn faster in a more natural environment, similar to the way that young babies learn to speak from the way their caregivers speak to them. Start slowly and work your way into more complex sentences as you go. Your students will show you how well they are understanding and learning by how well they respond to you. If you don't get any responses, it means that you need to pull back and speak more slowly and more clearly. If your students are chatting a lot and always answering your questions, you can feel free expanding and beginning to work on other types of speech or vocabulary.