From NFHS rules Appendix G - HS rules on Lightning
 

• When thunder is heard or lightning is seen, the leading edge of the thunderstorm is close enough to strike your location with lightning. 
• Suspend play for at least 30 minutes and vacate the outdoor activity to the previously designated safer location immediately.
• 30-minute rule: Once play has been suspended, wait at least 30 minutes after the last thunder is heard or lightning is witnessed prior to resuming play.
• Any subsequent thunder or lightning after the beginning of the 30 minute count will reset the clock and another 30-minute count should begin.