'Tenth class' = we assume this means as opposed to 'first class' or even second or third rate. ^_^;;;;

As far as we know, there aren't any such numerical classes for youkai. And since Hiei achieves 'Fifth class' after powering up in 'Hiei-chan vs. Sensui Again,' Tenth class is obviously worse. We figure it must go up from Tenth class to First class, and then maybe it starts backwards through the alphabet: Z class, Y class, X class...

Actually we've never heard of a youkai lower than D class, either, but no matter how you look at it, Hiei-chan has a long way to go.