I am using Vivo V9

NOTE: Please turn off your BATTERY SAVER mode to receive notifications, if you have enabled. After turning OFF the battery saving mode you'll receive notifications from Skooly.

Please try a few other steps too

Step 1: Goto "Settings" and tap on "More settings"

Step 2: In "More Settings", Tap on "Applications" 

Step 3: In the Applications, Goto "Installed"

Step 4: In the Installed apps, search for "Skooly" and tap on it

Step 5: In that tap on "Notifications"

Step 6: now slide the toggle parallel to "Allow notification"

Now you will be able to receive notifications from "Skooly" App