Starting a new answer, since it looks like my initial answer was a misunderstanding...
would indicate that the Student variable in the mat file is a 2D array. I would assume it is a 2D structure array although your code shows it being saved as a scalar structure.
would indicate that the Student variable in the current workspace is a cell array. Presumably, the cell array contains structures but why isn't it structure array like in the mat file?
So, it looks like you're replacing a whole row of a structure array. I'm unclear what problem you're having. Aren't the values in the whole row identical after your update?