How can I visualize that data for a cell is not applicable?

Let's say I have a data table of students:

Name   | Birthdate  | Exit of school | Average Grade
----------------------------------------------------
Alice  | 01.02.2010 | 23.10.2017     |          1.2
Bob    | 28.04.1990 | 08.10.2011     |          1.7
Charlie| 28.02.2015 |     N/A        |          1.5

Charlie did not exit school so far, so this is not available.

What are design options / considerations to show this?

I see the following alternatives:

  • Leaving the cell empty
  • Putting - (many options here) inside the cell
  • Putting N / A inside the cell
  • Filling the cell in black
  • Putting a diagonal line from lower left to top right in the cell

Are there any advantages / disadvantages of the options? Are there other options?

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