Hi Edit,

I would suggest not investing too much effort in trying to find a universal answer. I come from a linguistic environment that makes this formality distinction and even if you asked me how to address a Czech teacher or tutor, the best advice I could give you would be to just ask them. Some people will see an italki class at a lower level of formality than an interaction with a unknown person on a street or at a place of business and that is fine with me.

I was lucky to start my italki experience with language/culture combinations that pretty much ignore this distinction: Chinese,  and then French with a Canadian tutor. When I had a German class, I just waited till the tutor addressed me (as "du"), I would have been fine with either form of address and if she had not given me a clue, I would have asked her explicitly. In a few days I will have my first Portuguese class with a tutor from Angola,  and I am just ready to accept whatever choice she prefers. Conversely, I suppose most tutors are aware that other cultures and languages may not make the same formality distinction as theirs and will be ready to guide the student in this.

Boa sorte,

Michal