I recommend all my students (Básico 1, Básico 2, Intermedio 1) the book “Essential Grammar in Use”. Its author is Raymond Murphy, and the publishing house is Cambridge. http://books.google.com.pe/books?id=CxkhwIbvwOAC&hl=es&source=gbs_similarbooks
It is a book to practise grammar. It has theory and lots of exercises. It has explanations in Spanish, and it includes the answers, so you can correct the exercises yourselves.
This book can give you the extra practice that you need, because the exercises that you do in the Workbook are not enough.
It’s in our library, so you can borrow it from the school. Or you can buy it in many bookshops: http://libros.fnac.es/a34386/Raymond-Murphy-Essential-Grammar-in-Use-CDROM