I know of no site that has the
entire high school rule book (NFHS, more commonly called FED) for free download. The NFHS wants people to buy the rule book (they are available from
NFHS Order Page-Baseball. They are available for $6.75, although there is a minimum order of $10.00 (not including shipping). Of course, you could fill the minimum order by also getting the umpires' Case Book (same price), which will go into greater detail about some of the rules, and explain specific situations. In addition, Officiating.com has the book "
Baseball Rule Differences" for $24.95, which is a valuable tool for comparing different rule books.
This site (
http://eteamz.active.com/baseball/rules/) has some comparisons of OBR (pro) and FED (high school) rules, but not all the FED rules listed.
In the case that you mentioned, what happened WAS legal under high school rules (I am assuming that Pennsylvania uses the FED rule book). BigWI has already given the rule book reference (it is under
3.1.2, though
). This rule states that a pitcher may be returned to the mound only once per inning, provided the return does not violate the pitching, substitution, or charged conference rule. There is nothing to prevent a player from pitching, leaving the mound and playing somewhere else (or even leaving the game entirely), and then returning to pitch later in the game. Under high school rules, it would not matter whether the pitcher is a starter or not.
I believe that Mustang ball uses the OBR (Official Baseball Rules, published by MLB) as the basis for its rules. These are different in some areas than FED. If you cannot return a pitcher under Mustang Rules, this would likely be the reason why.