I would recommend making about a quart for 25-30 students, but the amount really doesn't matter.
1/2 quart water
about 2 boxes cornstarch
Put the water and food coloring in a large bowl and begin adding the corn starch and mixing. Eventually the mixture will get thicker; keep adding and stirring. You will know when you have Oobleck!
For good background information and a lesson plan see this University of Arizona page: http://sciconn.mcb.arizona.edu/oobleck/oobleck.html