It's really clear what each pose is, which I admire because on my faster sketches, my drawings tend to resemble scribbles more than anything else. That being said, they do seem kind of stiff. I notice you're not using lines of action, which have really helped me with my stiffness. Especially with really simplified sketches, you'd be surprised how much you can push a pose before it stops looking "real", and the line action helps you to see where and how to push it.
My first question is have you done the tutorial? Drawing a line of action can feel super awkward, but it really helps long term because it gives you a basis for cranking out those 30s poses and making lots of terrible drawings.
The second thing I recommend is trying even a 30m class if you haven‘t. Yeah, the 30s poses feel impossible at first. But if you‘re new to figure drawing, those poses are where pushing to find a line of action really pays off. If you take a line of action seriously, you‘ll find very quickly that your 30s poses improve the fastest out of anything. A longer class is not better, even a 30m class can be too long if you‘re new to figure drawing.
The third thing to try is mark up your classes afterwards. Just take a minute or two to pick the best drawings out of a timed group. Don‘t worry about the bad ones, just mark which ones are best. It doesn‘t do any good to focus on the bad drawings, classes are designed to produce a lot of bad drawings. What you want to do is focus on what worked, what you got right.