Students are unable to set their own Standard in their 'Preferences' tab. However, they will be able to use those Levels and Standards which are associated with any classes that they are enrolled into. If they are not enrolled into any classes, they will have the default 'Sight Reading Factory' Standard and customization option available to them. So, if you'd like your students to see a particular standard (NYSSMA, VCDA, etc.) you'll have to first create a class, enroll your students into that class, and then associate the Standards and Levels that you would like available to them.
After taking these steps, your students will now be able to see the Standards/Levels that you'd like available to them.