Two other reasons I can come up with for being easily distracted in a classroom setting is if one bored/unchallenged or if one has a learning difficulties that makes it hard to follow directions.
This! My DS once was in a class where he could use 5% of his brain power to passively follow the classroom instruction while doing other things that were of actual interest to him (this looked like distraction, until the teacher quizzed him and he got every answer right).
Other factors to add to your list are sleep deprivation, seasonal allergies and anxiety.
