Play-on for lower levels
The play-on can be used at any level of play. We do not teach it to Level 1 officials, because in youth games the offended team is typically less likely to be disadvantaged by a quick whistle than at the high-school level.
If you know all of the rules and mechanics regarding play-on, you are free to use this at any level.
Keep in mind that, despite confusion among some players and coaches, a play-on is not the same thing as a flag down (which is used at all levels of boys lacrosse in Minnesota).
It is at least illegal procedure to throw a stick (which could be a loss of possession or a 30-second penalty, depending on the status of the ball). This is one of those issues where you should probably not call a penalty the first time you see it; instead, try to educate the players and coaches. Make sure they know that throwing sticks is illegal, and make sure the coach knows that they have been warned. If it continues to happen during the same game, then a technical foul may be in order.
Note that it is unsportsmanlike conduct to throw the stick at the ball during play or to throw it at an opponent.
Coaches on the field
Coaches are permitted on the field only at halftime, to attend to an injured player, or to warm up a goalie. Any other time the coach is on the field is technically a conduct foul. Any other time the coach is on the field--including timeouts--politely remind the coach that he cannot be on the field.
If the coach is on the field and it is a problem (e.g., coach interferes with the play or the officials, coach is on the field complaining about a call), call the conduct foul.
Proper application of the conduct foul
Many officials mistakenly believe that a conduct foul is a 30-second penalty. It is not. A conduct foul by Team B is a technical foul: loss of possession to Team A, unless Team A is in possession, in which case it is a 30-second penalty.
Ball in back of crosse
There is no rule prohibiting a player from carrying the ball in the back of his crosse, and doing so constitutes possession. However, if the ball is stuck, that is withholding the ball from play. Seeing the ball defy gravity at any point or seeing the ball stay in the stick when the player attempts to throw or shake the ball out is grounds for determining that the ball is "stuck." Don't call the withholding until you determine that the ball was actually stuck, even if there is a lot of lobbying from coaches or players.