I used Luma Inks (concentrated liquid watercolors) for
20 years before I switched to tube watercolors. Luma's
are most likely too expensive for schools but they are
great for watercolors. If you use Luma's or other
liquid products be aware that most are fugitive. Luma
has a list of the colors that are light fast. Those are
the few colors I used in my painting. I suppose the
other companies have lists of their light fast colors.
Of course the fugitive nature of the colors may not
be an issue for student materials.
Woody