Student or Learner
I will send the offer by tomorrow.
Is this line is right
Its is fine. It mean that you will send the offer till tomorrow. Tomorrow is the deadline.
It means that you will send the offer till by tomorrow.
'till' is wrong. It applies to a continuous activity eg. I'll wait till tomorrow.
not a teacher
This is to certify that xxx has been working in our School from 1st April 2008 as teacher and has an experience of one year. We hereby state that his code and conduct has been satisfactory during his tenure with us.