As the title says, I would like to know your opinion:
In nearly every drawing tutorial, they starting with squares and circles to form a body. My art teacher says I shouldn't do that, but I'm confused.
Is this way to learn just ONE way, or really the best way to go - To achieve the best learning experience?
And is there a difference with the start of a figure between drawing from your imagination, or drawing in an akt session?
Its just ONE way and you need experiment with a few and it does not seem really like the wrong way to me. Ask him specifically why he has a problem with it as it sounds more like a personal bias of the teacher.
Your second question yes there is. In fact there are multiple approaches to both drawing from observation and from imagination. If you look at Vilppu and Michael Hampon as example they specifically try to force you to treat what you do in the life drawing room in precisely the same way you would do for drawing from imagination. For the circumstance it was important because Vilppu had animators draw in a completely different way in the life drawing room and it wasn't transferring across. That's why he says "we never copy the model"
You can however doing things like enveloping and using the negative space. For a methodology far more suited to observational figure drawing check out Anthony Ryder.
Now no way it better than the other and you do yourself a disservice not to play with all of them for they are all useful in their own way even from an animator or concept artist. Later on you can concentrate on what works best.
At the beginning though just draw. It is going to be difficult utilizing any methodology so just ease yourself in. No pressure.
setting up the act, in basic volumes is probably the only way to learn to understand the space, since it forces you to heavily abstract features.
Your art teacher is dead wrong, even if he/she prefers some other way of setting up/building form, since there's really no substitute for basic geometric bodies, as a tool and a reminder.
In time you'll learn to rely on spheres and cubes, less and less, but even DaVinci never abandoned them completely, in his sub-constructions.
I want to tell you that I am also used to do exactly same process like you to draw body shapes and I don't found anything wrong about that. Your teacher's opinion matters lot in this method but, you should tell her that this trick helps me to draw quick and accurate painting of human being.
I've never seen a good tutorial or any good book suggests starting from circles and squares. They always suggest starting from ovals (ellipsoids) and boxes (rectangular cuboids). That's a huge difference. Ovals and boxes have depth and orientation in three-dimensional space.
Note that there is infinite number of ovals that can fit into the same 2d ellipse contour.
Stating 3d orientations of main forms is extremely important because they reflect flexion/torsion of the spine, which is the key element in every gesture.