by Daniel Wolff.
The way we did this on our installation was to use groups and groupings. Each student was put into their own group and then a grouping was created for each assignment. When it was determined a student should complete an activity their group was added to the assignment grouping.
I don't see a way that moodle or any other LMS could assign students to specific assignments in an easier way as you would need to indicate to the LMS which students need to see what.