Bram Stoker called his book Dracula: A Mystery Story, and in the Victorian era, his story really was mysterious. There's so much horror and suspense leading up to the realization that Dracula is—gasp!—a vampire, and it knocked the socks off of the people reading it for the first time. Unfortunately, this is a case where too much fame can lead to a letdown. Your students know that Dracula is a vampire, and they might get very impatient waiting for the other characters to reach the conclusion they've known since page one.
Hey, can you tell that Mina is supposed to be pure? Did you know she's also really kind-hearted and good? Are you sure? The male characters only bring it up approximately five hundred million times in the novel, so it might be confusing!
In all seriousness, there are a lot of interesting themes in Dracula about purity vs. corruption, but the constant talking about Mina's wonderful goodness can get very repetitive and heavy-handed, especially for a teenage audience who wants to skip the moralizing and religiousness and get back to the blood-sucking and killing. Be prepared for this in case they start to nod off in the later chapters.