I have a Oreo Phone

Step 1: Open Settings

Step 2:Select 'Power Management' under Device.

Step 3:Select 'Auto-start Manager'

Step 4:Enable 'Auto launch' by swiping right

Step 5: Select 'Allow' from the Pop-up window.

Step 6:Press Back to save your Settings.

Option 2:

Step 1 : Go to device settings

Step 2: 

Go to Applications / Application Management and select "Skooly"

Step 3:

Select Notification or Manage Notifications

Step 4:

Enable Priority Notifications

Step 5 :

Enable priority setting and ensure the settings are same

Step 6:

If you can see "Categories" . Under categories enable skooly

Step 7:

Select categories and ensure the following settings are same as below