The code is checking the database value against 1 << spellInfo->School so it should match the spell schools defined in the code (SharedDefines.h). Or translated into bits and mask values you can use in the database:
SPELL_SCHOOL_NORMAL = bit 0 (0x00000001)
SPELL_SCHOOL_HOLY = bit 1 (0x00000002)
SPELL_SCHOOL_FIRE = bit 2 (0x00000004)
SPELL_SCHOOL_NATURE = bit 3 (0x00000008)
SPELL_SCHOOL_FROST = bit 4 (0x00000010)
SPELL_SCHOOL_SHADOW = bit 5 (0x00000020)
SPELL_SCHOOL_ARCANE = bit 6 (0x00000040)