Giving lessons outside of school in something other than his speciality
Question

                I work as a teacher and I teach more than one subject to people outside of school in areas that are not my speciality although I know them well. Is this halaal or haraam?.
Praise be to Allah.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.