Due to the teachers' requests, the new students added to your class will automatically show up at the bottom of your roster. :)
You'll notice their cell is green, for highlighting purposes. To remove this "new" status and let them sort alphabetically in your list, just Right Click on the cell, and "Remove New Status".
This is your choice. You can keep it how it is, or sort it how you like.
Here's a 45 second video if you need assistance.
Thanks.