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?

It happens once in a minute, twice in a week, and once in a year?

What is it?

Who makes it, has no need of it.
Who buys it, has no use for it.
Who uses it can neither see nor feel it.