You are able to permanently delete Notifications that were created by other users at your school - be careful when you delete (which is actually good overall website editing advice).   

As with Posts, there are two ways to delete events.

Method 1 – Delete Directly

Navigate to the Notification in edit/secure mode, click on gear icon and choose Delete Notification:

Make required modifications and save/publish as usual.

Method 2 – SiteGovern

If you have created a Notification but not yet published it, this is the only way to delete it.

In SiteGovern, go to Recent Contents and choose the content type (shown on the right) from the dropdown.Select Notification and then, from the list of recent notifications, delete the one you would like to remove:

Method 3 - SiteGovern > Scheduler

If someone else created a Notification, and you may remove it, this is the only way to delete it.

In SiteGovern, go to the Scheduler and choose Notifications. Expand the Notification section. Click on the three dots below "More". Select "Remove Article" from the choices. Press "Save". Exit SiteGovern.