In Western Europe, the social network is not as precisely layered as it is in China.
As a general rule, you call a superior "Sir" or "Madam". To start a letter, you generally write: "Dear Sir or Madam". It suits a wide variety of situations.
To a professor, you may say "Professor" — but "Sir" or "Madam" are never wrong.
I am not aware of specific callings for senior students.