Hello teacher Adriana, I know the situation can be frustrating, because yes, it happens, there are many students that arrive to this level with no knowledge of the language and as you just mentioned, it makes almost impossible to follow the program. In this case you would have to start from the beggining, try to give some examples of the book if it is possible, to ler them know some vocabulary or some other topics, but in grammar you would have to start with the basics. Sometimes we have students with more knowledge than others, that makes difficult to catch up for some students, but you can always divide them in different groups and/or make them help each other depending on their level.
Hope this can help you
Greetings