Gouache has been , traditionally , a graphic designers medium. It is
much more refined than tempera and offers true opacity. The good
stuff is very VERY expensive and the cheap stuff is no better than
tempera. I see little use to make the expenditure for good gouache
in the the school setting, especially elementary .
The last time I checked a good tube of black gouache was over $20.00
and those tubes are teeny tiny.
Don't even fool with the cheap stuff.