Dartmouth High School’s Harmonix had quite the surprise for the coaches of WGBH’s singing competition “Sing That Thing.”

The a cappella group’s debut performance — its first time ever appearing in a major competition — premiered on April 26 on PBS. Originally recorded in February, the group appeared on Episode 3 of the program.

The group performed a special “Michael Jackson Melody,” which referenced many of the King of Pop’s most well-known songs. Students also danced, and wore Jackson’s signature white glove throughout the performance.

The coaches had a hard time believing the group was marking its first competitive performance on stage before them. Annette Philip, from the Berklee College of Music, praised Harmonix’s five soloists.

“I wasn’t expecting that,” Philip told the group. “That was incredible. Are you sure this is your first time competing? If this is where you’re starting, you’re going to take the world by storm.”

She did offer some tips to improve: Perhaps slow down the show a bit to give the audience more time to get wrapped up in the lyrics and pick out their favorite Michael Jackson hits.

Jared Bowen, WGBH’s Executive Arts Editor and fellow coach, agreed. He suggested introducing crisper movement and interaction in Harmonix’s choreography.

“I think you have it down, so now it’s about how do you continue to evolve, and how do you make it even crisper,” Bowen said.

Anthony Trecek-King, the final coach and artistic director of Boston Children’s Theater, cautioned the group that towards the end, the show came off as a bit flat, and advised more inflection and group dynamics.

“Overall, it was great,” Trecek-King said. “I loved the energy.”

The three coaches each said the group was “Likely” to proceed to the final round of the show.

The coaches will pick two groups from each of the three divisions  — college, adult, and high school — to compete in the Grand Finale. A total of 18 groups are participating.

Episodes are aired Fridays at 8 p.m. on WGBH2. Click here to watch Dartmouth High’s performance on Episode 3.