Giving lessons outside of school in something other than his speciality

If you know the subject
well that you are teaching, and the students know that it is not your
specialty, then there is nothing wrong with you teaching them. But if you do
not know it well or you give them the impression that this is your
speciality, then it is not permissible, because that is deceiving them. The
student is looking for a specialized teacher and dealing with him on that
basis, so you should tell him the truth so that he will know what is going
on, otherwise it is deceiving him. 

Muslim (101) narrated that
the Prophet (peace and blessings of Allaah be upon him) said: “The one
who deceives us is not one of us.” 

And Allaah knows best.