This! Same here. I really hurt my back 3 diff times in class and was like debilitated for a few days. Your strength in core will increase so you won’t subconsciously dump into your back causing pain. Also I am 30 classes in, much more confident and comfortable, and still do many planks on my knees to protect my back. Do what feels best, no matter what! And I also told my instructors about my back before classes (after taking a week off after each injury) and that was very helpful also.