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.”"

Question

Answer

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

No pierdas la oportunidad de aprender un idioma desde la comodidad de tu casa. ¡Explora nuestra selección de profesores de idiomas con experiencia e inscríbete ya en tu primera clase!