By default, individual course scales should not be accessible to other users on the site unless they've been designated a site-wide "standard" scale, which is something only admins can do.

However, we've discovered that if you create a Grade Item (via Grades > Choose an Action > Categories & Items > Add Grade Item) and click on the "Scale" drop down menu, you see all the custom scales available on the site.

The steps to recreate are:

1) Create a custom scale for Course A as Teacher A
2) Assign that scale to a grade item in Course A.
3) Create a grade item in Course B as Teacher B
4) Add a grade item, and look at the scale menu; if the bug is there, then you'll be able to see the custom scale that should have been limited to to just Course A.

In looking through the code, I discovered the problem lies on line 60:

if ($scales = get_records('scale')) {

This query returns *every* scale in Moodle, both custom and site-wide, rather than just those for the course.