What exactly was the Night Class living on (and for how long) before creating the blood tablets? They obviously lived on campus while working on the tablets, and the Night Class has been at the academy for at least 4 years when the anime started.
So, wouldn't it be kind of dangerous to have them living that close to the humans with NOTHING to help them with their hunger for blood?
Most likely each other. we see later on that Vampires can survive on each other's blood such as when Kaname was drinking from Ruka (while Yuuki was peaking though the door) and later when Rido was resurrected he was feeding off Vampires too1. Also i seem to recall some moments between Senri and Rima when one of them was cut and the other drank from the cut.
However keep in mind that the Vampires of the Night Class aren't Level E's so they have a bit more self control. also one of the duties of the Guardians is to make sure that Night Class Students aren't feeding on Day Class Students who sneak out at night.
in the worse case scenario Kaname, as a Pureblood, can force their obeisance to not drink human blood. also remember that the Night Class is meant to be the foundation for coexistence between Vampires and Humans, as such not only are there school rules against drinking human blood (as Yuuki mentions to Maria) but also Kaname and Kaien would make sure that the only Vampires they admit to the school would be ones who can tolerate not drinking human blood, which at the beginning would be Vampires who would want the Blood Tablets to be successful
1: i suspect they were normal Vampires as some of the women seemed to be wanting Rido to drink their blood rather than wanting to drink his blood
