Line of Scrimmage
 

In high school can a passer cross the line of scrimmage and then retreat back behind of scrimmage and throw a legal pass. Please let me know and where do you find that rule?

Yes he can.

The rule is 7-5-1 and reads as follows:

"ART. 1 . . . It is a legal forward pass, if during a scrimmage down and before team possession has changed, a player of A throws the ball with both feet of the passer in or behind the neutral zone when the ball is released.
Only one forward pass may be thrown during the down."

Note that all that is required is for the passer to be in or behind the neutral zone.

Yes. Take a look at 7-5-1. For a forward pass to be legal, the following things need to be true:
1) scrimmage down (ie: not during a free kick).
2) Prior to change of possession
3) Both feet of passer must be in or behind NZ when pass is released.

The first forward pass meeting all three criteria is legal, the second one is not.

Note that part 3 only cares about the location of the feet at the time of the pass; nothing is said about where the passer was prior to the pass being thrown.

As a follow up to what others have said, the other rule sets (NCAA/NFL) specifically state that the action in your scenario is illegal by rule. NFHS does not prohibit said actions.