Spell.dbc/SchoolMask

Mask	IntMask	School
0x01	1	Physical
0x02	2	Holy
0x04	4	Fire
0x08	8	Nature
0x10	16	Frost
0x20	32	Shadow
0x40	64	Arcane

Version - 5.4.8 - SpellSchools ()

DBC Flag	School Name	Comments
1	SPELL_SCHOOL_NORMAL	Physical
2	SPELL_SCHOOL_HOLY	Holy
3	SPELL_SCHOOL_HOLYSTRIKE	Holy + Physical
4	SPELL_SCHOOL_FIRE	Fire
5	SPELL_SCHOOL_FLAMESTRIKE	Fire + Physical
6	SPELL_SCHOOL_HOLYFIRE	Fire + Holy
8	SPELL_SCHOOL_NATURE	Nature
9	SPELL_SCHOOL_STORMSTRIKE	Nature + Physical
10	SPELL_SCHOOL_HOLYSTORM	Nature + Holy
12	SPELL_SCHOOL_FIRESTORM	Nature + Fire
16	SPELL_SCHOOL_FROST	Frost
17	SPELL_SCHOOL_FROSTSTRIKE	Frost + Physical
20	SPELL_SCHOOL_FROSTFIRE	Frost + Fire
24	SPELL_SCHOOL_FROSTSTORM	Frost + Nature
28	SPELL_SCHOOL_ELEMENTAL	Frost + Fire + Nature
32	SPELL_SCHOOL_SHADOW	Shadow
33	SPELL_SCHOOL_SHADOWSTRIKE	Shadow + Physical
34	SPELL_SCHOOL_TWILIGHT	Shadow + Holy
36	SPELL_SCHOOL_SHADOWFLAME	Shadow + Fire
40	SPELL_SCHOOL_PLAGUE	Shadow + Nature
49	SPELL_SCHOOL_SHADOWFROST	Shadow + Frost
52	SPELL_SCHOOL_UNKNOWN	Invalid (Still listed in SpellMisc)
60	SPELL_SCHOOL_UNKNOWN	Invalid (Still listed in SpellMisc)
61	SPELL_SCHOOL_UNKNOWN	Invalid (Still listed in SpellMisc)
64	SPELL_SCHOOL_ARCANE	Arcane
66	SPELL_SCHOOL_DIVINE	Arcane + Holy
68	SPELL_SCHOOL_SPELLFIRE	Arcane + Fire
72	SPELL_SCHOOL_SPELLSTORM	Arcane + Nature
80	SPELL_SCHOOL_SPELLFROST	Arcane + Frost
84	SPELL_SCHOOL_UNKNOWN	Invalid (Still listed in SpellMisc)
96	SPELL_SCHOOL_SPELLSHADOW	Arcane + Shadow
123	SPELL_SCHOOL_UNKNOWN	Invalid (Still listed in SpellMisc)
126	SPELL_SCHOOL_MAGIC	Arcane + Shadow + Frost + Nature + Fire + Holy
127	SPELL_SCHOOL_CHAOS	Arcane + Shadow + Frost + Nature + Fire + Holy + Physical

I am not sure to how far back some of the extra schools date but my guess for many is 4.0 and higher although many may well go back to 3.3.5 and lower.
I'm sure the invalid ones were changed later to newer ones that are not listed (Ex. Radiant).