In the Cyberstone Classes and Teachers list in SharePoint, is the Teacher column a lookup to another list? Or is it just a text field?
In general, I think the rule you currently have on the Course field is close. You need to add a filter to indicate which teacher you want - I'm attaching a screenshot. The first value in the filter is the ID from the Cyberstone Classes and Teachers list. That is because in your Course dropdown, the value field is the ID - so the Course field in your form will be storing a number. The field we compare to in the filter is the Course field in the main data source.
We are saying:
In this list of many rows, called Cyberstone Classes and Teachers, I want the Teacher from the row where the ID is equal to the selected Course in my form.
Does that make sense?