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.

Follow your PSG's instructions. Your 1SG and Commander did not pull any authority away from your PSG by placing you in a course. Yes your course is your assigned place of duty, but this does not exempt you from following further instructions just because it is inconvenient for you. You are never out from under leadership in the Army. The SFC that released you from your class is not releasing you from your responsibility to follow your PSG. Think of it as a release back to your unit leadership. PSG's and squad leaders have many reasons for administering such instructions. If you are unsure of their reasoning, talk to your first line supervisor. Not every student in your class is in your platoon. Their mission and soldiers are different. Quit comparing yourself. If your PSG is keeping your from attending the assigned course, it would be another story.

Report as directed by your PSG. Refer to AR 600-20 for authority.

Some units have policy letters signed by the Commander that discuss duty and reporting requirements for Soldiers in local schools / classes, read your unit policy letters to see if the Commander has exempted Soldiers enrolled in local schools from certain duty requirements.

If your Commander hasn't signed a policy letter exempting you or other Soldiers attending local schools from duty then your PSG is certainly able to have you show up after class and perform tasks.

How long are you cleaning for? You need to pick your battles wisely and if you are only cleaning to say 1800 it sounds like a nine hour day before a lunch break. I am not certain that is worth pissing off the PSG. Much later and it is wrong. Also remember the PSG needs to allow you to eat in a timely manner unless the task takes precedence (few things in garrison do).

If I was in your shoes I would talk with your PSG and if it doesn't solve the issue then use the open door policy. Nevertheless, be aware that it might be a reason for him/her to keep you pass training time.