by Przemek Kaszubski.
I stand corrected on one thing - it IS possible to check multiple students' names displayed in a list when grading an assignment and then extending the deadline for all of them. If you have a group, you can use filtering to short-list the students first.
With quizzes there are individual User and Group-level overrides in the quiz administration settings. The former also allows to pick multiple users if needed.
Not the same as different timetables for different groups, but fairly close