Even though the catcher/fielder is ready to receive the throw, he cannot block the plate unless he has the ball in his glove. In other words, he can't stop traffic by holding up his hand. However, there are times when the catcher may have no choice but to block the plate. For example, if the batter is standing right on it and about to be hit by a pitch, then it is necessary for the catcher to get out of the way. In addition to these rules that all catchers must follow, they are also allowed several exceptions to them. For example, they are permitted to block the plate with their hands if they are receiving a thrown ball and don't have time to put on a mitt before the batter reaches base. They are also permitted to block the plate if they are certain that no one is out at home plate or another person is able to catch the ball before it hits the ground. Finally, catchers are allowed to block the plate if someone else is in danger of being injured by a thrown ball. In conclusion, yes, a catcher can block the plate in high school baseball.