A lot of our education users use Apple Configurator to set up multiple iPads, and from time to time, they encounter an issue loading Haiku Deck.
Whenever this has happened so far, it's been due to settings in Configurator that restrict 12+ rated apps.
Because of Haiku Deck's 12+ age rating, it will not be allowed by Apple Configurator installations with age restrictions. The only way to resolve this would be to contact the department in charge of the iPads and Configurator's settings, and adjust the age restrictions to allow 12+ apps.
For more information about why Haiku Deck has a 12+ age rating, check out our blog post here.