What does mean "Which I guess it pretty much is"?
The full text is: " We called the bus the Singleberry Express and sometimes the Singlebus, although Heller had once commented, “That thing looks like it’s taking mentally handicapped students to choir camp. Which I guess it pretty much is.”"

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The writer/author is agreeing with 'Heller' saying that what he said is correct. :)

Evden çıkmadan dil öğrenme fırsatını kaçırmayın. Deneyimli dil eğitmenlerimizden oluşan seçkimize göz atın ve ilk dersinize şimdi kaydolun!