Click on the MORE button and some options will display to you
Click on the Select Students
You can untoggle a student from writing the exam. E.g I don't want Fortune to write this exam all i need to do is click on the toggle button to disable him from taking this exam subject.
Once done, click on the close icon to close the modal and then click on update button to save the changes.
Once you have updated it, you can always go back to enable the student and save the changes again.