Diagnostic Test and Other Questions

Hello. I'm gonna start my part time job tomorrow. I'm gonna teach English (Grammar, Listening, Reading and Conversational) to an Elementary School Student and a Middle School Student here in Korea. I don't know how old they are. I just know they're currently in Middle School Elementary School. I want to give them a Diagnostic Test to gauge their knowledge about English already. My problem is I don't know what test to give them. I don't have the resources. Well of course I have the internet but I did not bring any English book with me because I thought I will just be teaching lessons on Conversational English. Do you guys know or have any good sources for Diagnostic Exams? I don't want to just randomly get from the internet because it might be too difficult for them. Also, can you recommend me good grammar/reading//writing books for Elementary and Middle School level. It will be my first official tutorial so I really want to do good. Thank you!