According to J-Cast, readers of Weekly Shounen Jump vol.42 pointed out that the descriptions of the inhalation of helium gas in manga Sket Dance chapter 105 is extremely dangerous. Shueisha made an apology in the Weekly Shounen Jump official website that they were unaware of the safety.

The apology comment by the Weekly Shounen Jump editors

In the chapter, Bossun and Himeko change the pitch of their voices by inhaling helium gas directly from the gas tank. Vendors of helium gas warn that such an action can result in suffocation. In 2007, a high school boy inhaled helium gas from a plastic bag and died of asphyxiation. The toy gas cylinder used for changing the voice pitch contains 20% oxygen, but the normal helium tank used for blowing balloons doesn't contain oxygen.

Source: J-Cast