The 2015 season saw the Broncos take on a new challenge, as they were moved up to Class 6A due to new IHSA regulations. 2015 was also a special year for the Montini community, as it marked the 50th year of existence for Montini Catholic High School. The first two games saw the Broncos pull out wins against perennial playoff powers in Maine South and East St. Louis. After a 38-7 victory over St. Rita, the Broncos went on a roll, scoring at least 35 points in five out of their last six games to finish with a 9-0 regular season record for the fourth time in school history. The Broncos faced the Wauconda Bulldogs in the first round of the Class 6A playoffs and prevailed 35-7 in a game played in wet conditions. Montini then faced an old foe in the second round, steamrolling the Kaneland Knights 42-7 to advance to the quarterfinal round, where they defeated the DeKalb Barbs 49-14 to reach the semifinals against the Prairie Ridge Wolves.
The 2015 semifinal game against Prairie Ridge is considered to be the greatest game ever played at Montini. In a driving snowstorm, the Broncos scored first, only to watch the Wolves come back and take a halftime lead. Prairie Ridge led 27-13 in the third quarter when the snow stopped falling and the Broncos took over. Montini scored 22 unanswered points, including a fumble recovery scoop-and-score by linebacker Jake Kapp, and prevailed 35-27 to reach their seventh consecutive state championship game. Facing the Crete-Monee Warriors at Husky Stadium in DeKalb, the Broncos scored 28 unanswered to turn a 15-10 deficit into a 38-15 victory. Not only had the Broncos captured their sixth state championship, they had also done what no Montini team had done before and finished the season with a perfect 14-0 record.
08/28 vs. Maine South–W 20-17
09/5 @ East St. Louis–W 24-14
09/11 vs. St. Rita–W 38-7
09/19 vs. De La Salle–W 38-0
09/25 @ Leo High School–W 41-0
10/2 @ St. Joseph High School at Elmhurst College–W 42-0
10/9 vs. St Francis–W 27-8
10/17 @ Fenwick at Morton West–W 49-7
10/23 @ Marmion Academy–W 35-0
10/31 vs. Wauconda–W 35-7
11/7 vs. Kaneland–W 42-7
11/14 @ DeKalb–W 49-14
11/21 vs. Prairie Ridge–W 35-27
11/28 vs. Crete-Monee at Huskie Stadium–W 38-15