1
I’ll take the keys in case you won’t be here when I arrive.
I’ll take the keys in case you aren’t here when I arrive.
As with other conjunctions such as before, after, as soon as and unless, when in case refers to the future, we normally use a present tense, not will or shall.
2
In case of younger students, the lessons are shorter.
In the case of younger students, the lessons are shorter.
Use in case or in case of when preparing for something that may happen: ‘Take an umbrella in case it rains.’ ‘In case of fire, use the stairs.’
In the case of means ‘when talking about a particular person, group or situation’: ‘In the case of younger students, the lessons are shorter.’