It's an interesting question. It's a time-honored tradition in Russian to use вы and sometimes to not use it. Sometimes the borderline is very obscure, and when this borderline is not observed this may cause tensions.
Not using it intentionally can be often understood as manipulation, which is another interesting aspect of it.
In case with student to professor address - вы is a must, and using имя+отчество is a must - if we are talking about any institution starting from public school to grad school, i.e. any 'official' academic environment.
One exception that I am thinking about would probably be something like language courses when your tutor can be younger or your peer, so the range of address can be anything from a short name to longer name to имя-отчество (and the educational environment is very informal here, so this borderline is very thin and sometimes doesn't exist at all),
but Господин/Госпожа is very official and is never used when addressing professors. In fact this is used often to establish the distance between the parties, when such distance is necessary.
One has to remember that Господин/Госпожа hasnt been used in Soviet times (товарищ was the universal address), and is now often viewed with suspicion by many.
So students don't address professors as Господин/Госпожа - that would be too official and cause too much a distance between them, so in this case the Anglo-Saxon use of Mr./Ms is not the same as Господин/Госпожа. In the Russian academic environment use of имя-отчество is traditional and doubtless.
to dtrq, PROFESSOR means just преподаватель ВУЗа или колледжа in AmerEnglish.
to those who watched Pozner-Bezrukov interview - think about how they addressed each other, and you see that it's an important problem in Russia...