Vampires are traditionally seductive by nature, and thus in many interpretations, their bites give a feeling of sexual satisfaction to compliment it. A certain popular movie featuring vampires does this, but I'll pick another, more traditional example that might fit the thread more due to the subject matter.
There's an anime you may have heard of called Black Blood Brothers. I won't get into the details about the show itself, but one of the major points of the show was that biting represented sex without being all "KIDS, COVER YOUR EYES". The reason for this stems from the vampires necessity to feed. They are not all innately evil creatures, and as such, feeding has that effect on the ones who are fed off of to make a mutual relationship rather than a parasitic one. Blood bags are viable, but they are open to contamination, aren't as fresh, and doesn't taste as good.
Really, I like it that way as well. The other way would make it seem as if all vampires are "evil", even if it is for necessity. There'd be next to no way to convince people to have their blood sucked other than coercion, and since blood bags exist, the school would probably ban the practice completely, as it would probably be seen as bullying, harassment, and invasion of space. The other way just fits these vampire's natures more, and it doesn't put a label on the vampires unless they suck the humans dry.
