The email address that Alerts are sent from can be controlled in Site Manager > Site Administration > Site Modules > Alerts.
At the top of the page are two fields, where you can specify a custom 'From' email address and label.
Enter a name in the "Display From Label" field. This name will be shown either instead of or in addition to the email address in recipients' inboxes.
The actual email address goes in the "From Email Address" field. Any replies to the Alert message will be directed to this address. The address in this field must come from a domain that your school controls (i.e., an Alert from Finalsite Academy must come from an "@finalsiteacademy.com" email address).