If you have recorded the correct Skype username and the student is not receiving your call attempts, it is possible they have "Only allow Skype calls from contacts to ring on this device" enabled in their account.

Here's what they need to do:

  1. Select your profile picture.
  2. Select Settings.
  3. Select Calling.
  4. Toggle Only allow calls from contacts to ring on this device to OFF.

Your students will also want to make sure their Skype status is set to Active. More information information can be found here: https://support.skype.com/en/faq/FA34699/what-are-the-possible-skype-statuses