a delayed response, here -- i was joyfully engaged on a backpacking trip over the weekend!
kathleen, depending on the target cost of the finished paint sets, i go for a double primary palette (warm and cool of each primary), plus maybe one or two earth tones (burnt umber, burnt sienna) and maybe a green such as viridian (which quickly gets the point across that some raw-looking colors may actually be quite good at playing with others). there's also nothing like a cadmium orange, so i try to include that, too. once i had a lot of quinacridone gold on hand, for some reason, which my students also loved. so my answer to your question is, "it varies", but i make sure it's an array of 8-10 you-can-mix-yourself-to-anywhere-from-here colors.
linda, just to clarify, the small pro-art palette i'm referring to actually has 20 wells (now that i'm home i can include a picture, linda, if you send me your direct email address) -- more than enough room to include whatever colors you wish. i find that air-dried tube paints re-wet quite readily for around 6 months, and are still reasonably cooperative for about 2 years, which i consider to be a good window of usability for the cost of the set (--especially considering that the paint can be replaced and the palette can be used indefinitely). i do use Daniel Smith tube paints, preferentially, but i also find that the Van Gogh tube paints (not dry cakes -- i don't think VG even makes them) are a good, economical choice for participants who would otherwise not be able to afford a paint set at all.
have a great time with your students, linda. they are so lucky to have you as their teacher!