Mathew - No ... and I'm hoping that wasn't intended as a dig.
The Cowboys' defense my junior year was actually one of their worst (18.3 ppg allowed). Our defense was much better my senior year (11.9 ppg), but the 1994 Cowboys were better known for their offense, which scored 419 points (the best mark in modern-era Class A history at the time).
Sarah - No and No. Dillon came close from 2000-02, when they allowed 7.5 ppg, 11.4 ppg and 7.7 ppg, respectively. Butte Central has never really had a dominating defense. The Maroons have only held opponents under 10 points per game once since 1980, when they allowed 8.2 ppg in 1996.
Rick - You're not far off, at least when it comes to the time frame. Sidney had back-to-back seasons of allowing fewer than 10 ppg (8.9 ppg in 1992 and 7.4 ppg in 1993). The Eagles had allowed 10.8 ppg in 1991.
The correct answer, surprisingly enough, is Ronan. The program has fallen of tough times over the last decade and a half or so (to the point where the Chiefs will be playing at the Class B level this season), but Ronan was one of the best teams of the late 1980s and early 1990s.
1989 (9-2, runners-up to Sidney): 9.3 ppg allowed
1990 (3-5, missed playoffs): 10.0 ppg
1991 (7-2, lost in semis to Sidney): 9.4 ppg
1992 (8-2, lost in semis to Sidney): 7.1 ppg
1993 (6-3, lost in quarters to Sidney): 8.7 ppg
Ronan came within a single point of having five consecutive seasons of having a an opponents' ppg of under 10.0.
Other than Miles City, the closest any team has come to matching Ronan since then has been Billings Central. Here are the Rams' opponents' ppg since 2007:
2006 - 9.5 ppg
2007 - 4.6 ppg (the best Class A defensive team since 1980).
2008 - 11.3 ppg
2009 - 6.9 ppg
2010 - 10.7 ppg
Here are the top 5 teams in terms of defensive ppg since 1980:
2007 Billings Central (12-0, state champions, 459-55 PF-PA): 4.58
1983 Glasgow (9-2, state champions, 260-59 PF-PA): 5.36
1998 Hamilton (11-0, state champions, 397-61 PF-PA): 5.55
1999 Laurel (11-0, state champions, 487-62 PF-PA): 5.64
1984 Sidney (9-1, state champions, 228-61 PF-PA): 6.10
Miles City's best defensive effort in terms of points allowed was in 1984, when the Cowboys held opponents to 6.70 ppg. This ranks them 10th.