There really isn't much of an official distinction going on between extras and those with one line
Er... no, it's a very explicit distinction, consisting of very rigidly defined job descriptions as established in the pay schedule of SAG/AFTRA in the US, and BAFTA in the UK, and their equivalent unions/guilds/orgs in various nations of the world.
The pay schedules of these groups provide what constitutes the legal definition and criteria of what a particular type of performing job entails, for purposes of everything from payroll, to one's placement in the opening/closing credits of a film or television series, to one's entitlement to residuals, and much, much more. It lays out exact distinctions between a Series Regular, a Recurring Character, a Day Player, an Extra, etc, etc.
The notion that one could get a film or television degree of any kind, without learning about these most basic and fundamental definitions, which must be followed at every level within the industry, is pretty much all the evidence one needs that one has received a very piss-poor education, and should seek some form of legal remedy that involves compensation.