Underwood was born on August 19, 2007, the son of Jay and Beverly Underwood. He attended Belleville High School, and played football for the Tigers under coaches Dejuan Rogers and Calvin Norman.[1][2] In 2021, Underwood's freshman season, he led his team to a Michigan High School Athletic Association (MHSAA) state championship. In the title game against Rochester Adams High School, he completed 12 of his 21 passing attempts for 284 yards and five touchdowns, while also rushing for 62 yards and a touchdown.[3] In total for his freshman season, he threw for 2,888 yards and 39 touchdowns with just four interceptions, while also adding six scores on the ground.[4] For his performance on the season, Underwood was named the MaxPreps National Freshman of the Year and the Michigan Associated Press Division 1-2 Player of the Year.[5][6][7]
In 2022, his sophomore season, he passed for 2,762 yards with 37 touchdowns, while rushing for 632 yards and eight touchdowns.[8] Underwood led Belleville to an undefeated season and a second consecutive state championship. Following the season, he was named the MaxPreps National Sophomore of the Year and the Michigan MaxPreps Player of the Year.[9][10] In 2023, his junior season, Underwood passed for 3,329 yards and 44 touchdowns with just three interceptions; while also rushing for 199 yards and seven touchdowns. He led Belleville to a third consecutive state championship appearance, but lost to Southfield A&T High School, snapping a 38 game winning streak throughout his high school career.[11][12] Following his junior season, Underwood was voted the Michigan High School Football Player of the Year by the state media and awarded the Michigan Gatorade Player of the Year.[13][14]
Entering his senior season, Underwood was rated as a five-star recruit, the No. 1 quarterback and the No. 1 overall player in the class of 2025.[15] Underwood held offers from nearly every division 1 school, such as Louisiana State University (LSU), Alabama, Ohio State, Florida, Penn State and Michigan.[16] On January 6, 2024, Underwood initially committed to play college football for the LSU Tigers.[17][18] On September 13, 2024, Underwood broke two MHSAA state records, for the most career passing touchdowns and most career total touchdowns, eclipsing the previous state records of 124 passing touchdowns and 151 total touchdowns.[19][20] As a senior, Underwood failed to make a fourth consecutive appearance in the state championship game, losing to powerhouse Detroit Catholic Central High School in the regional championship.[21] In total as a senior, he passed for 2,509 yards, throwing 32 touchdown passes and six interceptions, while also rushing for over 600 yards and an additional six touchdowns.[22][23]
Following the season, he was awarded the Michigan Gatorade Player of the Year, voted the Michigan High School Football Player of the Year, and named the Michigan Associated Press Division 1-2 Player of the Year; winning all three for a second time in his career.[24][25][26] He also won the 2024 Michigan Mr. Football Award.[27] Underwood finished his four years at Belleville with a 50-4 record, two state championships, and was credited with 12,919 all-purpose yards and 179 total touchdowns; including passing for 11,488 yards and 152 touchdowns (all four being MHSAA state records).[28]