[grantsrc]

Sorry to disagree with you here but according to the officials manual published by the fed, you do not throw a flag for the first sideline warning. The second you do, and they get 5 yards, the third you throw the flag and they get 15 yards. See below:

According to the NFHS Officials Manual, you do not throw a flag on the warning. (2004 and 2005 Officials Manual, page 22, under Controlling the sideline and team box):

II. First Violation by Team.
A. Results in a sideline warning.
B. Covering official sound whistle, give proper signal, then report infraction to referee.
C. Referee will give proper signal and indicate offending sideline.
D. Covering official echo signal (No. 15) while verbally informing coach of warning. Communication also should include reminder of distance penalties of further violations.
E. All officials record time and period warning is given.

III. Second Violation by Team.
A. Covering official will drop penalty marker, sound whistle, report infraction to referee.
B. Referee will give approved signals and umpire will assess distance penalty.
C. Covering official will verbally inform and remind coach that each subsequent offense will result in 15-yard penalty.
D. All officials record time and period penalty is assessed.

IV. Third and subsequent Violations.
A. Covering official will drop penalty marker, sound whistle, report infraction to referee.
B. Referee will give approved signals..."