I am using Samsung (Galaxy S8, S8 Plus, Note 8)

(The following screens and settings are applicable for all of the above-mentioned devices)

Step 1: Open your phone settings.

Step 2: Select "Apps" on the list of settings.

Step 3:  Now select "Skooly" from the list of apps

Step 4: In the App Info, click on "Notifictions"

Step 5: Now the status must be OFF, now click on the toggle to turn it ON 

now you will be able to see something like this,

Now you must be able to get notifications