  We analyzed the photometry of 20038 cool stars from campaigns 12, 13, 14 and
15 of the K2 mission in order to detect, characterize and validate new
planetary candidates transiting low-mass stars. We present a catalogue of 25
new periodic transit-like signals in 22 stars, of which we computed the
parameters of the stellar host for 19 stars and the planetary parameters for 21
signals. We acquired speckle and AO images, and also inspected archival
Pan-STARRS1 images and Gaia DR2 to discard the presence of close stellar
companions and to check possible transit dilutions due to nearby stars. False
positive probability (FPP) was computed for 22 signals, obtaining FPP < $1\%$
for 17. We consider 12 of them as statistically validated planets. One signal
is a false positive and the remaining 12 signals are considered as planet
candidates. 20 signals have orbital period P$_{\rm orb} < 10$ $d$, 2 have $10$
$d < $ P$_{\rm orb} < 20$ $d$ and 3 have P$_{\rm orb} > 20$ $d$. Regarding
radii, 11 candidates and validated planets have computed radius R $<2
R_{\oplus}$, 9 have $2 R_{\oplus} <$ R $< 4 R_{\oplus}$, and 1 has R $>4
R_{\oplus}$. 2 validated planets and 2 candidates are located in moderately
bright stars ($m_{kep}<13$) and 2 validated planets and 3 candidates have
derived orbital radius within the habitable zone according to optimistic
models. Of special interest is the validated warm super-Earth EPIC 248616368b
(T$\rm_{eq} = 318^{+24}_{-43} \, K$, S$_{\rm p} = 1.7\pm 0.2 \, S_{\oplus}$,
R$_{\rm p} = 2.1\pm 0.1 \, R_{\oplus} $), located in a m$\rm_{kep}$ = 14.13
star.
