I have a wizard that specialized in Conjuration. Evocation is among his prohibited schools. There is a spell called "Kelgore's Fire Bolt" (PHB2) that is marked as both Conjuration and Evocation [Fire]. According to the rules, can this specialist wizard learn and cast the spell or not?
Dual-school spells appeared only in Player’s Handbook II and Dragon Magic, and so those are the books that explain them (the explanation is the same in both).
In all cases, treat these spells as if they belonged to both schools simultaneously. Effects that prevent a spellcaster from accessing one school of a dual-school spell prevent all access to that spell. For example, a specialist wizard cannot learn a dual-school spell if either of the spell’s schools is one of his prohibited schools.
(Player’s Handbook II pg. 95, Dragon Magic pg. 60)
So no, your wizard cannot learn or cast Kelgore’s fire bolt as a wizard spell. You could cast it from an item, or using wish or something similar, or take levels in another class that gets access to it, but as a wizard, without something exceptional, it’s not an option for you.