I needed some basic icons for the different spell schools - so I got creative (well, you might disagree ;)):
All of the eight spell schools (abjuration, conjuration, divination, enchantment, evocation, illusion, necromancy and transmutation) drawn with a used, painted look. I choose the colors based on [https://www.dndbeyond.com/spells/detect-magic#c6](this post on the dndbeyond.com Forum), and I think the fit rather well.
The icons are in jpeg format and are coming in five sizes, ranging from 1000x1000 to 32x32 pixels.
You can grab your copy at: https://www.vttassets.com/asset/icons-spell-schools or directly from this post.