After some time of teaching German language you will certainly familiarise yourself with different German language textbooks. You will like some of them more and some others less. At some point you will also notice that new textbooks are being regularly published and replace older ones no matter if they were good or bad. For example, I liked “Stufen” (Klett) very much and worked with it some 15 years ago. No one is using that textbook any more. “Stufen 1” contained a very good phonetics part which was very useful to students. I am still using it for my classes.
For children (6-9 years old) I would recommend “Spielerisch Deutsch Lernen” (Hueber). It is in my opinion optimal – not too easy and not too demanding. It will leave you much time for playing different games or learning songs.
“Ideen” (Hueber) seems to be a right choice for younger teenagers (10-15 years old) because they are a special group of learners and teenagers learn differently. This textbook considers all specific traits of these learners.
“Deutsch.com” (Hueber) is very suitable for older teenagers (15-19 years old) because it contains interesting topics.
For adults (regular and intensive courses) I am using “Delfin” (Hueber). It is well organised and leads the learner from A1 to B1 level.
Those were my recommended German language textbooks for the beginner courses. Since textbooks are only a basis for a good course, I also recommend that you use describing pictures in your classes. I have also some recommendations for your classes with older teenagers in a state school. You could also create your own exercises and worksheets, Also, working on your students’ vocabulary is a very important task.