Mark All Read in Label

[scottymeuk]

Select mark all read from the labels on the left

[JohnW2]

There is no 'flag' signifying that a message has been read -
there is one to signify "unread" however.
All the displays you see of your mailbox are really the
result of a search through your mailbox for messages
which bear the particular labels.

Besides, you can add the button "Mark as read"
by enabling the appropriate experiment in the Labs.