Student or Learner
Why did we use in instead of on?
He is going to camp in mountains next week.
Surely you've learned by now that prepositions are sometimes not logical. Yes, he is on the surface of the mountain side and not "in" the mountain like a vein of silver, but that's just what we say.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.