If you are taking lessons in a foreign language and you only study in class, you will make progress, but this depends on your teacher, the course, and how much you remember. The teacher will decide on the style and content of the lessons and on the books and other materials you use. If you don’t want to worry about such things yourself, that’s fine.
Another way to approach lessons is to see them as a starting point for your own studies, an opportunity to ask about things you’re sure about or don’t understand, and to practise what you’re learning on. Some teachers will be happy to answer your questions, others might prefer to stick to the curriculum and might tell you that you don’t need to know or worry about the answers. For language learners who like to understand how languages work this can be frustrating, but there are plenty of other sources of information – dictionaries, grammars, websites, native speakers, etc.
It’s great if you can find a teacher and class that suits the way you like to study, but if you can’t, try to make the best of what’s available. The same applies to textbooks and other language learning materials – it’s unlikely that you’ll find one that is perfect for you, but you can get useful information from whatever materials that are available.
The key is to decide what you want and need to learn, then use the resources available to you as best you can. You don’t have to learn everything included in textbooks and lessons, but rather focus on the parts that are most useful and interesting to you.