With an authoritative server, there is no way of being 100% fair without exposing the player to latency. There is always the possibility that the player made a sudden move that didn't reach the server before the ball passes the paddle, resulting in the player failing even when he should have hit it. But if the ping is low enough and 100% fariness is not required, you could be fine.
If 100% fairness is required and no latency is allowed, trusting the clients is the only option. This should be pretty straight forward. When the ball hits the player's paddle on client A, it tells about it to the others. When the ball hits the player B's paddle, client B again tells the others.
If the connection is very good, 100% fairness is required and the server has to be authoritative, it is still possible to make it using the lockstep protocol. This will introduce some latency but it might be ok with a proper connection. This was actually used as the first networking solution for Doom. In a nutshell, the game is divided into small, say 10ms, steps. On each step the client sends to the server any new player input instead of processing the input itself. At the end of the next step the server sends all received inputs back to the clients. On step 3, both clients process the inputs given on step 1 and so the games are exactly the same. If the server or the client hasn't received an input by the step it should have it, it locks until it's received. Hence the name lockstep I guess. A longer explanation can be found for example here and here.