Your code does not work because you are trying to apply one index onto multiple variables. MATLAB does not have such a command that applies one index to all members of a comma-separated list, nor is it likely to in the near future (that would be a major change in how commas and lists are interpreted).
a.students(1).grades = [7 8 9];
a.students(2).grades = [6 7 8];
a.students(:).grades % this returns multiple separate variables, not one array.
a.students(:).grades(:) % how can we apply one index to multiple arrays?
Basically your code is doing something a bit like this (pseudo-code):
[7 8 9],[6 7 8](:)
with the expectation that the colon index would be applied to both members of the comma-separated list.
