Unless your school absolutely wants a hard paper test based assessment, you might be better off doing a simple verbal test. I did this when I had to select students for our city's advanced English students program. Look at the material in their books from the previous year. Since most of it is usually question and response, you can ask them the questions and assess their ability in the answers. But make sure that it seems like a natural conversation and secretly record the conversation with each student on your cell phone for review later. (If you have a grade sheet in front of you and stop talking to write something down it can make them nervous.) If the student seems capable of discussing other topics, follow them off their tangents to assess what they have learned outside of the school curriculum.
There are also a set of questions all Korean students learn at hagwons or have been learning since kindergarten, "What's your name" "Can you tell me about your family?" "How old are you?", that many students can answer fairly quickly and confidently. Start with these questions to make them comfortable, but this will help as sort of the "control" questions like a lie detector test. If the student can continue to form answers to more difficult questions at the same speed and accuracy, you can give them a higher score. If not, you can figure out where you can start your curriculum.
For first and second graders without books, just ask them simple things like, "What's this?" while holding a simple item like a book, pencil, etc., or "What color is it?" If they seem capable of more, start asking them some of the questions from the third grade or fourth grade books, or easy ones like "What's your favorite color?"
I would also suggest using the afterschool classes to bolster what they are learning in their regular English classes. Perhaps there is a language aspect you would like to elaborate upon, or a game you think would be interesting but didn't have time to play in class. This will save you planning time and be more useful to the students and your co-teacher. I'm not sure about your school, but even our school's higher level students can't readily form the sentences in the books, so introducing new material and hoping it sticks is an exercise in futility.
I hope my suggestions can work for you,
Good Luck
