"Sunshine, daisies, butter mellow,
Turn this stupid, fat rat yellow."
He waved his wand, but nothing happened. Scabbers stayed gray and fast asleep. "Are you sure that's a real spell?" said the girl. "Well, it's not very good, is it? I've tried a few simple spells just for practice and it's all worked for me. Nobody in my family's magic at all, it was ever such a surprise when I got my letter, but I was ever so pleased, of course, I mean, it's the very best school of witchcraft there is, I've heard - I've learned all our course books by heart, of course, I just hope it will be enough -- I'm Hermione Granger, by the way, who are you?”
(Harry Potter and the Sorcerer's Stone)
There is seems to be a relative clause as she is in Her book displays the fine sceptical in-telligence of the scholar she is (CGEL,p1045). But I suspect there is some implicative meaning in OP’s there is. For example, there is Ø in the world, etc. Does ‘there is’ really mean what I’ve said?
Or there is is a set complement for the very best school of witchcraft (CGEL,p1101) and means ‘there is Ø among the witchcraft schools.
Or is it mean it’s the very best school of witchcraft [there is one among witchcraft schools]. And it refers to one.