Before School-Sul

Before School-Sul, sometimes referred to as Before School-Sul or BS-Happy, is the fourth official sub-unit of boy band Before School. Before School-Sul consists of five Before School members, the leader Daeho, Kyung Woo, Seung Ho, Min-su, and Seon.

2011: 꿀단지(Sunny) and PajamaParty
By 2011, all of the Before School members had already been confirmed to be a member of at least one subgroup except Byeong Na. Before School-Sul consists of every previous member from Before School-IL except JU, who is replaced by Kyung Woo.

On May 30, 2011, K-Sims Entertainment released the official announcement of the subgroup through NewsPlain. The subgroup's first EP, 꿀단지(Sunny), was released on July 18, 2011. The music video of their first single, "꿀단지(Sunny)" was released on that same day.

On the day before their debut stage, the subgroup held their first sign fan meeting to celebrate the success of the sales of 꿀단지(Sunny) .On the morning of July 20, over thousands of fans lined up for tickets to the fan meet, overcrowding the streets. Close to 10,000 copies were sold by the first week of release. According to the Music Industry Association of Appaloosa Plains, the EP sold 27,122 by the end of July and was ranked fifth in the monthly charts.

Before School-Sul performed their second single, "Pajama Party" on August 1, 2011 on KBS's Popular Songs. The music video was released on August 4, 2011. "Pajama Party" promotions only lasted a month, and promotions for the EP officially ended on September 7, 2011.