Milkweed (Asclepias and Gomphocarpus species) is poisonous but while some people state that they are not allowed to be grown in school grounds, no evidence can be found of this ruling.
There are two reputable sources on this topic. The first document available is Plants in New Zealand Poisonous to Children.
It refers to Swan plants (Gomphocarpus fruticosus and G. physocarpus):
“These two shrubby plants are so similar that they can be regarded as the same for all practical purposes. Like all members of the Asclepias family the tissues are full of poisonous white milky latex, so even the bladder-like fruits should not be eaten.”
The second document is “Safety in pre-school centres: plants to avoid” and does not contain swan plant. Milkweed such as swan plant is not on that list of plants because of its educational value in teaching young children about life cycles. It is also used as an introduction to scientific experimentation.
Children are unlikely to eat or try to eat milkweed because it tastes so disgusting. If you have ever had the sap on your fingers and then touched your fingers to your mouth you will know the the taste is vile.
By ensuring supervision of children at all times around the plant, or by using a large open-topped cage around the plant so children cannot reach the plant material, there should be no concerns. And (of course) by being responsible and teaching responsibility!
Many schools in NZ, and early childhood education centres too, have the plants growing and there have been no problems reported.