  We study the spectral classification of emission-line galaxies as
star-forming galaxies or Active Galactic Nuclei (AGNs). From the Sloan Digital
Sky Survey (SDSS) high quality data, we define an improved classification to be
used for high redshift galaxies. We classify emission-line galaxies of the SDSS
according to the latest standard recipe using [Oiii]5007, [Nii]6584,
[Sii]6717+6731, H, and H emission lines. We obtain four classes: star-forming
galaxies, Seyfert 2, LINERs, and composites. We then examine where these
galaxies fall in the blue diagram used at high redshift (i.e. log([Oiii]5007/H)
vs. log([Oii]3726+3729/H).We define new improved boundaries in the blue diagram
for star-forming galaxies, Seyfert 2, LINERs, SF/Sy2, and SF-LIN/comp classes.
We maximize the success rate to 99.7% for the detection of star-forming
galaxies, to 86% for the Seyfert 2 (including the SF/Sy2 region), and to 91%
for the LINERs. We also minimize the contamination to 16% in the region of
star-forming galaxies. We cannot reliably separate composites from star-forming
galaxies and LINERs, but we define a SF/LIN/comp region where most of them fall
(64%).
