From
www.ndhsaa.com/athletics/football/football_regulations (which I might add, was written incorrectly in the online .pdf resource guide for officials):
After the first half, whenever the point differential reaches 30 or more, the clock will continue running when...
A. The ball goes out of bounds.
B. A forward pass is incomplete.
C. After a touchdown and during the try (unless it reduces the lead to less than 30 points).
The clock will be stopped on all other situations as specified in rule 3-4-4. Review official's timeouts 3-5-7.
Note: The use of this rule does not preclude rule 3-1-3, which reads: "A period or periods may be shortened in any emergency by agreement by opposing coaches and the referee. By mutual agreement of the opposing coaches and the referee, any remaining period may be shortened at any time or the game terminated."
I can handle the change of possession stoppages, in this situation. The one that really seems out of place is continuing to stop the clock for the marking of chains on a first down. Granted, it may be only a 10 second deal on a larger than normal gain, but string five first downs together on a drive, and you've added a minute that should have been running. Doesn't seem like the amount of "mercy" intended by the rule.