- thou ( ты ) implies close relations, not intimate, but quite friendly, transcending from official co-working into personal realm and safe zones. Camaraderie.
- you ( вы ) implies somewhat cold official relations. You work together in some area, but other than that you keep "proper" society-normal communications level of civilized strangers.
Compare it to full/diminished forms of personal names, like Николай/Коля, Мария/Маша. Depending on persons relations different names would fit better.
So, some societies may like to stress their "brotherhood", "we are one gang".
While other societies may stress how they are not pushing their peers into "safety zone" of each other, how they are focused in the token activity and do not "push their nose" in you personal activities outside of the scope.
There is no one true answer, this point happens to ignite holy wars among native speakers too. Somewhere sharing "safe zones" would be frowned upon, somewhere it would be explicit distancing form others.
Personally I had that weird experience with my father's peer, who complained to my father I was informally addressing him via thou and simple name, breaking the proper boundaries. So I started calling him properly, via you/n+p. Few days later he complained again, about my weirdiness. Guess like when his "appreciate my position and age" ego was satisfied, the alienation in proper addressing forms started bugging him in turn. Or maybe he was just moody person. Anyway, in that interaction I switched to staying on "society approved" polite safe ground ever since.
One more situation, especially around women, is that there is a trait referring elder persons by explicitly polite means, like using "вы" or even using name+patronymics instead of name alone. Some persons feel like polite ways of addressing is intentionally or not stressing their aging.
вы
while talking to a person they never met before for the first time (although one could easily switch to usingты
later in the class, e.g. after working in pairs for a while). So, as pershabunc
's answer, the professor most likely wanted to give an informal touch to the class - however, I doubt that all the students actually found that comfortable in their heart. – Vadim Landa Aug 14 '17 at 11:11