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. :)

집에서 편안하게 언어를 배울 수 있는 기회를 놓치지 마세요. 경험 많은 강사진을 살펴보고 지금 바로 첫 수업을 신청하세요!