Thinking outside the box on this; a simple triangle could represent Maslow's hierarchy of needs.
I can't think of anything that would represent it better, that can also be captured and conveyed in such a simple icon.
A simple triangle maybe with a few horizontal lines depending on its size would be a well thought out icon. It may be a little obscure but that's even kind of a good thing, could give the feel of an exclusive joke for the well informed. Its also a topic many teachers should be aware of.
Something like this:
or whatever you wish!
Further to Bakabaka's comment, here are some variations that you can use to represent both essential and desirable separately.
I'm not sure if the additional line here creates too much noise.
Fairly easy to see the difference:
Hope I've helped in some way. :)