If you hit save and add another, or save and back you will not have this option to fill across unless you turn on Multiple event scoring. Here is how you do it:
Go to your mathematics grade book.
Click on Display options
Click on Event Display
Scroll down until you see "Event Score Entry". Select Use multiple skill event scoring screen. Click Save.
Type the score for that event in the first white box. It will automatically fill across. Click Save when finished.
You can then go back in to your Event Display Options to change back to single event entry if you want to in order to be able to select the no count and missing options.