I am really confused about this. My teacher tells me that after "in case" I always have to use subjunctive and this is the only acceptable form. For example : "Take a coat in case it should get chilly tonight." instead of "Take a coat in case it gets chilly tonight." My problem is that I can't find this rule in any of the grammar books I have and what is more my coursebook's answer key never uses subjunctive after in case. My hunch is that using subjunctive after in case is old-fashioned, but I am not really sure given that it doesn't appear anywhere. Please help me understand this, thank you!
The subjective is optional and not required after "in case". It is correct to say "Take a coat in case it gets cold."
As noted in comments, "should" is a modal and not the subjuctive. The subjuctive would be "Take a coat in case it get cold." (Which most speakers would find strange.)
 Take a coat in case it should get chilly tonight.
 Take a coat in case it gets chilly tonight.
I think your teacher is confused. The idiomatic "in case" does not license subjunctive clauses (we can't say *"Take a coat in case it get chilly tonight"), and in any case the presence of "should" rules out any possibility of the clause being a subjunctive.
In both examples, the preposition phrase "in case" is introducing a declarative content clause functioning as an adjunct of implicit purpose, where the meaning is Take a coat in order to avoid getting cold if it gets chilly tonight.
The only difference between the two is that  infers that it becoming chilly tonight is less likely than in .