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Twiddler
MOD
(On A Trope Odyssey)

An animated children's movie I saw in the 90's. It was about these spiders who wore clothes and went to school and played in a band. The smallest spider was the underdog/hero of the movie and played a piccolo. The others wouldn't let him join the band. The spiders go inside a human home, are spotted, and are captured in a glass jar, except for the smallest. He has to get the others out before they run out of air. Unfortunately, he ends up outside, and has to get back inside the house first. He tries climbing a waterspout, but it's raining and he's washed away. When the rain stops, he tries again and gets to a window, then goes in through a gap in the window. The jar with the other spiders is on that windowsill, along with a bunch of other stuff. Small-spider finds a way to break the jar, by pushing a baseball, which topples a ruler, which hits the windup mechanism on a Godzilla toy, which walks forward and pushes the jar to the edge of the sill, where it's easily toppled. After everyone has gone back to school, they all play a cover of "Itsy Bitsy Spider".
Edit from 9 years later: Solved! It was "The Real Story of Itsy Bitsy Spider" (aka "Spider Junior High"), part of "The Real Story of..." series
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Edited by Twiddler