I don't think it's particularly old fashioned, just very slangy and not often used.
The big cheese
The big kahuna
The head honcho
These are all the same, and in AmE I think I hear "head honcho" the most. Maybe there's another even more popular (or at least more often used) expression that I can't think of right now.
(not a teacher, just a language lover)
Student or Learner