To un-tick a box, using what we call "typpex" in French is OK, but don't try to redraw the empty box!
To fill a test with letter inside boxes, darken the box entirely.
To fill a box without letters inside, any significant writing near the center of the box should be sufficient.
Maybe make tests and add some sample from the previous real tests with students to show them what is considered as being ticked and what is not.