Go to System Settings > SMS Setting
Then select any SMS gateway from top of the page which you want to enable on your School then enter Auth Key, Sender Id and select Status and then click on Save button.
Custom SMS Gateway-
Also you can integrate any URL based SMS Gateway using our Custom SMS library.
Please contact to support to get Custom SMS Gateway integration guide.
Here enter Gateway Name, select Status (Enabled/Disabled) and then click on Save button.