I am using Nexus 5X/6P (Android 8.0)

Step 1: Go to "Settings"

Step 2: Tap on "Apps and notifications"

Step 3:  tap on "Notifications"

Step 4: tap on "Notifications"

Step 5: In the list of apps, search for "Skooly" and tap on it

Step 6: If the toggle is in "OFF", turn it "ON"

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

Step 7: Go back to "Apps and notifications" and tap on "App info"

Step 8:  In the list of apps, search for "Skooly" and tap on it

Step 9: In "Skooly" now tap on "App Notifications"

Step 10: Make sure the Toggle is "On"