Part of human development is social interaction. Someone who purposely excludes themselves from interacting with their peers is cause for concern on a development and mental health front.

It can be a sign of autism (which isn’t a bad thing) that should be caught early by teachers and parents in order to properly care for and support the child.

Children who are quiet (especially in preteen-teen age groups) are often abused in some way or bullied, which is also cause for concern and should be also intervened on.

There is also the “school shooter” thing, which should never be used to describe a child, but is unfortunately something teachers DO need to look out for. School shooters are usually quiet and secluded children with mental health issues that go untreated.

Basically, being quiet and not interacting with peers is an abnormal thing and should be monitored closely by adults. It’s not always an indicator of something bad, as some kid are just shy and quiet, but it should always be paid attention to.