What gets whiter the dirtier that it gets?

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?

Matthew leaves home in the morning to go to school. At the moment he leaves the house he looks at the clock in the mirror. The clock has no number indication and for this reason the boy makes a mistake in interpreting the time (mirror-image). Just assuming the clock must be out of order, Matthew cycles to school, where he arrives after twenty minutes. At that moment the clock at school shows a time that is two and a half hours later than the time that Matthew saw on the clock at home.

At what time did he reach school?