When rendering a Symfony2 form in a twig template I often get caught out when applying css classes to a label.
Label attributes are handled differently from a forms attribute and this often leads to confusion.
Adding css class attributes to a Symfony form field is done like so:
As you can see the parameter attr is supplied with a key of class. This is not the case when adding css class attributes to labels.
Not only is the attr parameter named differently but its also the third parameter of the label control:
So the parameter you need is label_attr. The second parameter is the label value. Set this to null if you want to use the label value defined in the form class.
You can find out more about using labels in twig templates by reading the Symfony2 documentation