To start with, an invoice can not be completely be deleted but can be archived. This means the the invoice would not be visible to both you the school and the parents of your school, we used the term archived because to us the invoice is still present but has just been excluded. 
Archiving A Previously Generated Invoice
Now to archive an invoice can be done in 3 simple steps.
1. After you must have been logged into your schoolable account, click on setup and then click on students as highlighted in the image below.
Use the search bar to find the student you wish to delete generated invoice for.

2. When you have seen the student you want to  delete generated invoice for., click the 3 dots right beside the date. 3 options would come up, to view, edit, delete. Click on view as highlighted in the image below. 

3. Search for the invoice you want to delete, click the 3 dots and proceed by clicking Archive. once  you click archive the invoice would not be visible to your school and the parent so be sure not to archive the wrong invoice