My first is in tea but not in leaf
My second is in teapot and also in teeth
My third is in caddy but not in cosy
My fourth is in cup but not in rosy
My fifth is in herbal and also in health
My sixth is in peppermint and always in wealth
My last is in drink, so what can I be?
I’m there in a classroom, do you listen to me?

"I never was, am always to be,
No one ever saw me, nor ever will
And yet I am the confidence of all
To live and breathe on this terrestrial ball."

Who makes it, doesn't tell it.
Who gets it, doesn't know it.
Who knows what it is, doesn't want it.

What is it?