Revealing mistake: When Truman is first discovered escaping on the boat, they go to a camera shot of his face. If you look at the water, the boat is not moving at all. They use a similar shot later in the scene after the storm. You can also tell the background is the same as the large wall that Truman crashes his boat into. (01:19:05 - 01:25:25)
Revealing mistake: If you look in Truman's car's mirror when he is backing up from the traffic jam, wires can be seen wrapped around his car.
Chosen answer: Clearly, in his media interviews, Truman's best friend, Marion, indicates that he did, indeed, grow up with Truman since they met in elementary school. One could extrapolate that his classmates did, as well. A sudden change of actor from one day to the next would be too much of tip-off to Truman. As actors leave the show, there is a story along with it. His father is presumed to have died, and returns later. I'm sure others move away. The main characters (Truman's wife, parents, close co-workers, and other key figures in his life) also appear to live "in world" and have devoted their entire lives to the Truman project. It is not made clear about the extras, though.
Michael Albert
It's kinda hard to believe though that a little boy (Marion) is faking it all so well for all these years since his childhood without raising any suspicion.
He probably didn't get to know it until a late age. In the bus, a little girl was confused and almost tell her mum that there was Truman, so a kid can't keep up the secret. I'm pretty sure they don't know it. But on the other hand, when he says he wants to be an explorer all the classmates freak out, as if they knew it would put in danger the show. So maybe the classmates and even the best friend had been selected after making sure they wouldn't mess up and reveal the secret.