  We study the content in S-bearing molecules of protoplanetary disks around
low-mass stars. We used the new IRAM 30-m receiver EMIR to perform simultaneous
observations of the $1_{10}-1_{01}$ line of H$_2$S at 168.8 GHz and
$2_{23}-1_{12}$ line of SO at 99.3 GHz. We compared the observational results
with predictions coming from the astrochemical code NAUTILUS, which has been
adapted to protoplanetary disks. The data were analyzed together with existing
CS J=3-2 observations. We fail to detect the SO and H$_2$S lines, although CS
is detected in LkCa15, DM\,Tau, and GO\,Tau but not in MWC\,480. However, our
new upper limits are significantly better than previous ones and allow us to
put some interesting constraints on the sulfur chemistry. Our best modeling of
disks is obtained for a C/O ratio of 1.2, starting from initial cloud
conditions of H density of $2\times 10^5$ cm$^{-3}$ and age of $10^6$ yr. The
results agree with the CS data and are compatible with the SO upper limits, but
fail to reproduce the H$_2$S upper limits. The predicted H$_2$S column
densities are too high by at least one order of magnitude. H$_2$S may remain
locked onto grain surfaces and react with other species, thereby preventing the
desorption of H$_2$S.
