Players running onto court withoput being beckoned

[Dudly]

Had a game last night and a simular situation. Freshman boys. I blow the whistle for a foul, while going over to report it (we'll be shooting 2 FT) red coach sends a sub to the table. The kid taps the table and runs onto the court and the kid he is replacing runs off. Had to get the sub off the court, get the other kid back onto the court, report the foul then continue with the game. Shot the Ft's, got the subs in. The kid who was trying to sub was giving me the "what did I do wrong" look. Luckily, red's coach was explaning to him how to sub in and out. No malicious intent here, just excited kids. This entire game could have been chalked up to teaching opportunity.