school of -- in the opinion of the curators a work by an unknown artist working in the studio of the named artist.
circle of -- in the opinion of the curators, a work by an as yet unidentified artist closely associated with the named artist but not necessarily his or her pupil.
style of, follower of -- in the opinion of the curators by an unknown artist who is contemporary, or nearly contemporary, with the named artist and is greatly influenced by his or her style.
after -- a copy by another artist of a known work by the named artist.