Joss Whedon has confirmed that anyone in the vampire's line can be referred to as their sire. Darla sired Angelus, Angelus sired Drusilla, Drusilla sired Spike, therefore, Drusilla, Angelus and Darla can all be referred to as Spike's sire, as we saw in 'School Hard'. It could also be taken to mean that Spike considers Angelus his sire because, while Dru did the actual deed in making him a vampire, Angelus was the one to teach Spike what it means to be a monster, so Spike could consider them both his sires.
In reality, though, they changed their minds between S2 and S5 and decided Dru being Spike's sire made for a better back story and so fanwanked the answer.
Though, it's worth pointing out that Angel never once confirmed he was Spike's sire, we just had Spike's bragging to go on.