  We developed a comprehensive model atom of Sc II-Sc III. Abundances of
scandium for a sample of eight unevolved A9-B3 type stars with well-determined
atmospheric parameters were determined based on the non-local thermodynamic
equilibrium (NLTE) line formation for Sc II-Sc III and high-resolution observed
spectra. For the Sc II lines, the abundance differences between NLTE and LTE
grow rapidly with increasing effective temperature (Teff), from slightly
negative at Teff = 7250 K to positive ones of up to 0.6 dex at Teff = 10400 K.
For Sc III in $\iota$ Her, NLTE reduces the line-to-line scatter substantially
compared to the LTE case. The NLTE abundances of Sc in our five superficially
normal stars are consistent within the error bars with the solar system Sc
abundance, while the LTE abundances of the late B-type stars are greatly
subsolar. NLTE reduces, but does not remove a deficiency of Sc in the Am stars
HD 72660 and Sirius. Based on our own and the literature data, the Ca/Sc
abundance ratios of the sample of 16 Am stars were found to be close together,
with [Ca/Sc] = 0.6-0.7. We propose the Ca/Sc abundance ratio, but not
abundances of individual Ca and Sc elements to be used for classifying a star
as Am and for testing the diffusion models. We provide the NLTE abundance
corrections for ten lines of Sc II in a grid of model atmospheres appropriate
for A to late B-type stars.
