school? If they get paid, how much per class? If they get other benefits, what are they? These are likely to be different depending on the size of the school and affluence of the area, so I'd be interested in hearing about that too.
If they are purple or below, they are expected to help with no exchange.
Help means I teach the class and lead. The blue is my demo partner. He walks around and helps the students technically as I do.
We never have blues or purples lead or teach on their own. They are only expected to assist.
Browns and blacks can get paid depending on the average rate per class in your area. And that is only if they teach on their own. 
In 
I was an assistant/coach at my last school. Helped during classes/tournaments and taught classes when my Professor couldnt be there. I got paid nothing but I got a lot of one-on-one time with my Prof to make sure I knew the material. To me, thats invaluable.
I formally teach as needed (a few classes a month-and don't pay any monthly or seminar fees). I feel that is a very generous arrangement I grew in to. I don't know what the other two assistant instructors have going, as I have never inquired, but I think the only ones getting paid are the striking instructors who don't participate in the BJJ program.
My kids used to get to train for free and I got an extremely discounted price, plus all the personal instruction/help I wanted/needed . Not bad considering I had 3 kids training and I was in the gym every day.
We usually pay them in synthetic Collaterialized Debt Obligations (CDOs) and credit swaps.
10$ per hour for kids classes, free tuition and seminars
Sex
30 a class....but I taught 6am class for years...most instructors not happy to teach at that time....I loved it...also free tuition obviously
Purple belt.. I just get to train for free but help a lot with coaching and extra time with white belts to help them improve.
Brown belt, I don't pay and my son goes for free.
shen -
We usually pay them in synthetic Collaterialized Debt Obligations (CDOs) and credit swaps.
Tranches is where it's at!
When I used to teach, I'd get 50/class minus membership fee at the end of the month. If for some reason I earned less than membership fee, they'd just give me the month free.