Django: 'LeagueAdmin.inlines' must be a list or tuple

The error is pretty clear -- inlines should be a list or tuple, not a class. Use

inlines = [TeamsInLeague]

or

inlines = (TeamsInLeague,)

The Django admin reference page has an example of a model with one inline item: even in that case, you need to make inlines a list.

So instead of what you have currently, use inlines = [TeamsInLeague].