Yes, that happens to me also. It's the ratio of chair height to table height. Measure it at home and then with a chair and table at normal height. I often take a firm cushion so that I sit higher than the standard chair height.

Usually quilt shop classes are folding tables set up. They are too high for me so I bring a bean bag type cushion to put in the chair. It doesn't get squashed and forms to my bottom. I bought the 'beans' at Walmart and made one to suit me.

Thanks everyone for your thoughts. I have some good ideas now. The chairs are not adjustable they seem to be standard chairs

I've pretty much decided that I'm not comfortable sewing anywhere but at home, so for the most part I don't. It's not worth the back ache. If there's a class I really want to take, I bring a pillow to sit on, and that helps with the chair height. Some ladies bring their own chairs.

One interesting thing many in our guild have figured out after doing a Zoom class, is that it's nice to be taking a class from home so you don't have to drag a machine and supplies and your set up is one you are used to--down side is the social interaction is little bit reduced. And the teacher must be well organized!

I've found that a lot of chairs at shops and retreats make my back hurt because they don't slope down slightly in the thigh area, and I always end up sitting hunched over. I just bought a new office "task" chair with this feature and it is SO comfy. I don't feel like I'm sliding off at all, but I do find it much easier to sit straight.

I got risers for one of my tables at Bed, Bath and Beyond that were adjustable. I got just the right height with them.

I agree that for me the back ache problem sewing away from home at quilt shops, guild workshops, or retreats was the chair. I solved it buy bringing my own rolling adjustable task chair with no arms with me whenever I sew away from home. It fits perfectly in the back seat of my van, seat of chair to seat of van with rolling legs in the air. I would like to take credit for this great idea but alas, a dear quilty friend brought her chair to one of our retreats and a light bulb simply went off in my head. One of those "Now why didn't I think of that" moments. Now quite a few of us bring our own chairs.