I would say: 'you attend school'. 'to attend to something' is used when you take care of/deal with something. You attend to work or you attend to a case/a matter etc.. you attend to children... but you don't attend TO school. (you don't deal with the school, you're just present during the lessons).
And as for unable/not able. I think that's swings and roundabouts. You can use both.