National Federation rules are generally employed across the board in high school athletics. National Federation official baseball rules differ in some significant ways from Major League rules.
Aluminum for Everyone?
Under National Federation rules, aluminum bats are permitted. The primary reason is that aluminum bats are more durable than wood, and will not need to be replaced as often (thus saving schools money).
Innings and Outings
As is the case with Babe Ruth rules baseball, most games last seven innings during the regular season, while playoff games are generally nine in division one. (Lower divisions tend to play seven innings, even in playoffs.) Again, this is often left to the state and division to decide.
While it is widely assumed that a coach must list all available substitutes on a lineup card prior to the game's first pitch, this is not necessary under National Federation rules. It is merely (strongly) suggested that coaches list available substitutes to avoid conflict and to prevent disputes and game delays.
What separates high school baseball from the Majors is a greater emphasis on sportsmanship. For example, a catcher "talking trash" to a batter, though widely accepted in the big league (see: Varitek vs. A-Rod), is not allowed in high school and usually results in an ejection/suspension. Additionally, a player may not blow on a baseball to make it roll fair/foul, another practice that often turns up on the Big League blooper reels.
Watch Your Mouth?
In Major League Baseball, a pitcher may not go to his mouth with his pitching hand while his foot is on the rubber; this immediately results in a balk (or a ball, if there are no runners on base). In high school, however, a pitcher is allowed to go to his mouth provided he is "distinctly wipes" his hand on his uniform pants before touching the ball. In pro ball, a pitcher must be granted permission by the home plate umpire to go to his mouth at any time-a right that is usually only granted if playing conditions are especially cold.
No Block Parties
It is common practice to block home plate in Major League Baseball-a tactic often used by the likes of Jason Varitek and A.J. Pierzynski. However, this is strictly prohibited in high school baseball. It is considered obstruction for any player to block a player from reaching a base he is trying to reach without possession of the ball. If a player beats the throw, he beats the throw.
Do you want to learn how to give baseball signals? Learning how to communicate via signals will help you and your teammates strategize without giving your gameplan away.