As the SDUI admin of your school, you have the option of downloading all "not yet registered" accounts again. You can find this function in the "Administration" area. To do this, go to "More" in the footer of the SDUI app and click on the "Administration" tile.
Here you land on the overview page of the administration area. In the screenshot you can already see the tile to "download not activated accounts". You have the choice to download the activation codes either as PDF or alternatively as Excel.
NOTE: The activation codes are only relevant for people who have not yet registered in Sdui. The activation code does not replace the password. If the person has already registered in Sdui and just can no longer log in, you would need to reset the password. Instructions can be found in the linked article: "How do I reset another user's password?"