Faithful projective representations of symmetric groups

Maybe it's worth adding a reference to a modern treatment of Schur's work in book form, which might be easier to read than the original paper of Schur (note that I'm unrelated to the authors):

P.N. Hoffman and J.F. Humphreys, Projective representations of the symmetric groups. Q-functions and shifted tableaux. Oxford Mathematical Monographs. Oxford Science Publications. The Clarendon Press, Oxford University Press, New York, 1992.

See Schur's original paper

Schur, I. (1911), Über die Darstellung der symmetrischen und der alternierenden Gruppe durch gebrochene lineare Substitutionen, Crelle's Journal 139: 155–250. (EuDML)