From gomanga:

Twenty five years ago, the alien Givers came to Earth. They gave the human race the greatest technology ever seen--four giant towers known as Ladders that rise 36,000 miles into space and culiminate in space stations to power the entire Earth. Then, for reasons unknown, the Givers disappeared. Due to the unique alien construction of the Laddertop space stations, only a skilled crew of children can peform the maintenance necessary to keep the stations up and running.
Back on Earth, competition is fierce to enter Laddertop Academy. It is an honor few students will achieve. Robbi and Azure, two eleven year old girls who are the best of friends, are candidates for entrance into the Academy. They will become entangled in a dangerous mystery that may help them solve the riddle of the Givers...if it doesn't destroy the Earth first!