Softball substitute
 

If a High School player substitutes for another player at bat and is not the DH, does that substitute player have to play in the field? If so, do they have to play a complete inning? For example, can the substitute player go in hit and play one out in the field, then the orginial player return to the field for the remaining of the inning?

In NFHS Rules (high school), there is no requirement that a player remain in the game for a certain number of out or innings, e.g. they could bat and then the original player or another substitute could play defense.

Or, they could be entered to bat and immediately be replaced again without even taking a single pitch. Of course, that would burn one entry for that player, but the point is there is no limitation on how long a player stays in the game once entered.

The only limit is on how many times a player may return to the pitching position in the same inning, (once), but even this only applies to returning to pitch, not leaving or re-entering the game.