How do I determine my appointed place of duty when given conflicting guidance?

Report where you PSG ordered you to report.
Inform your PSG that the additional duty time takes away from your study time, and respectfully request reconsideration.
If the PSG insists, respectfully request to use the 1SG's Open Door Policy. Personally review it before you go see your PSG, and I'd have a copy in my pocket. If the policy requires anything written, I'd have that filled out also.
(My commander and I adopted a form the Marines used when we deployed with them. Essentially, a form that gave space to write the issue, reference (AR, policy, etc), and what outcome the Soldier desired.)
Inform your Instructor of the additional duty requirements, as there may be a local policy that attendance at that course requires units excuse Soldiers from additional duties. The instructor may be able to contact your unit for "clarification."
If you have to use the 1SG open door policy, stay squared away to avoid providing the PSG any opportunities for reprisal.
Always stand at parade rest, yes SGT, no SGT, at right time and place, etc., everything you should be doing already.

This sounds like the SSG doesn't know his place. Your COMMANDER made it your place of duty. That is enough. The regulation is the fact that the E6 answers to that O3.
I recommend you tactfully remind the SSG that the CO assigned that class as your daily place of duty, not as a side job. Your duty is to learn what this class is teaching. If it is something like FAST class then you will have a 4187 backing you up with the CO's signature. If he doesn't buy that then you have the right to utilize the open door policy with your CO.
This happens too much, people need to realize that you don't over task people, especially when they are conducting training. Depending on the class you may have attachment orders which definitely will trump whatever SSG Power Trip is saying.

1. Your PSG rank is immaterial SSG or SPC- they are treated exactly like they were wearing SFC rank on there collar. 2. If your PSG says REPORT after class- then you have been given an ORDER to be there after class. Your CO/1SG put you in the class- I seriously doubt you have any thing in writing from them to be excused from any thing other than class. And unless this extra PSG time does not interfer with your class you have no gripe. So unless you want to be an E3, I strongly suggest you pull your head out of your 4th POC and do as ordered.