Browsing the D&D Beyond spell list it is clear that each spell has an associated symbol related to the spell's school:
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Doing a quick search for more information on this I have found:
- a reddit thread that points to the presence of the same symbols in the 5e DMG as well as in some of the magic items and references the 3.5e PHB (which I am unable to verify)
- an enworld thread claiming that these symbols go back at least as far back as AD&D 2e PHB referencing a Dragon 302, page 51 as well (I can't verify any of these either)
It seems clear that these symbols have been used across editions. I am mainly interested in the following:
- What is the earliest instance of usage of these symbols?
- Did the symbols hold some meaning beyond representing a spell school? (for example, being part of a language or script)
- Was there a resource that actually mentioned/explained them? (the post mentioning Dragon 302, page 51 seemed to hint at this)
Update: Although I was hoping for an official explanation, as Quadratic Wizard's answer details, such an explanation probably does not exist. I have accepted his answer for tracking down and detailing the origins but I would like to leave a note of appreciation for Kryan's answer as well for being a very good source for the actual meanings of the symbols. I find both are well worth reading.