We know as you said, Dumbledore was hoping he would have some control over his friends, but I am sure Dumbledore knew better. I think part of Dumbledore's plan was to help Remus with feeling like he was worth something, that his life had some meaning.Originally posted by Carole Lupin:
It just means he was either the most responsible Dumbledore could find, or that he never got caught.
I think Remus probably suffered knowing with the anti-werewolf sentiment and the registry, he wouldn't be good for much when he graduated. Being made a prefect could have boosted a fragile self-esteem or at least Dumbledore hoped.
As far as getting caught, though I am not sure to what you are referring, we know they never got caught running the grounds. Dumbledore didn't know the other three were Animagus and running around once a month with Remus in werewolf form until the end of PoA when Sirius confessed all.