There's some other problems with that scene though.
Wood says they're 10 minutes away from the school. They're driving down a road with no street lights, turns, or stop signs. You have to figure they're going at least 60 miles an hour. This puts them a minimum of 10 miles from the school. The school would have to be walking distance from Buffy's.
This means a vampire (Spike) has a sniffing range of at least 10 miles. Spike says he will find Buffy by following her scent. Also, because Spike appeared to be walking, a vampire would have to be able to move incredibly fast (at least 60 mph) in order to make it believable Spike would travel all that distance to warn Buffy rather than just walk over to the school to check out Xander's problem.
Personally I don't think the writer considered all this. I think she just stuffed all that stuff in there for dramatic affect without worrying about the believability factor.
I'm going to choose to excuse what I can and accept the rest blindly.
I actually thought of that, but then i realized it is no mistake... because xander's story isn't happening at the same time... think about it... xander goes with the girl, he sends willow a message saying he got lucky, which everybody obsiouly interprets as something is eating xander's head... spike goes to warn buffy, while xander is probably being knocked out and taken to wherever the seal was (i'm still not sure it was the school's) spike and buffy go rescue xander while he is being sacrificied... so it really isn't a mistake...