Trying to remember an Anime Name

Ataru and Lum are among the students at Tomobiki high school involved in preparing for the upcoming student cultural festival. Some of them grow suspicious that their days have been repeating. To validate the theory, they are all sent home, instead of sleeping overnight like they had been. All the students end up back at the school, unable to leave the city limits. They board a Harrier jump jet and fly upwards into space. They see that their entire city is on the back of a giant turtle, in the otherwise empty void of space. The students investigate the school, believing it to be the source of the strange happenings. Seemingly impossible events transpire, like Ataru chased by his own reflections in an infinity mirror.