Overview
District's firewall may be blocking emails that are sent from School Loop (e.g., LoopMail notifications).
Solution
In such cases, users with District Admin role in School Loop should whitelist the School Loop Mail Server IP address in their District's network firewall to avoid emails being blocked by a spam filter: 35.227.129.160/27
Also, you can whitelist the *.schoolloop.com domain from which all emails are being sent (e.g., no-reply@securemail.schoolloop.com, system@schoolloop.com).
The above settings may require you to involve your network/IT team to implement them since they fall out of School Loop's control.