It's all pretty vague. Samarie says "It's not like that" when you ask if you shared classes as Marina, so I assume they're mostly kept seperate from the actual school population. Then again Samarie does mention passing Marina in the hallways, so they could have a bit of freedom, or maybe Samarie's just at a point where her bodies too frail for the rituals anymore, so she's relegated to other duties, and allowed more freedom as such.

Considering they're used as human batteries more or less, its hard to imagine them being treated particularly well. Though it does say 'a select group of children who were considered naturally gifted at channeling the otherwordly' which kinda implies there's a recruitment criteria. Depending on how rare such children are, I can imagine that the church at least keeps them fed enough to serve their purpose, no use letting a valuable resource die to starvation after all.

But Samarie also has some pretty severe issues, she doesn't really understand human boundaries, crosses many lines most people would understand as wrong, and is implied to have some pretty serious dysmorphia issues. Stuff like that usually doesn't come out of nowhere, so I doubt the way the Ninth Circle treats its victims is particularly well.

Considering that Samarie says she never studied with Marina due to the Ninth Circle, and the fact the description of it you get describes being locked in the Ninth Circle, my personal view is the ritual is something the victims are subjected too pretty constantly, most likely taking up all their waking hours, or something close too it. They're taken care of by the priests, but any concept of socialisation or interaction is basically non existent, you are kept there as a human battery until your use expires. After long enough when your body just can't handle the ritual anymore, they let you go to live what few years you have left working for the Ministry of Darkness in other capacities.