Can I tutor on sundays?

I teach piano lessons. I get paid everyother week. But some of the students clearly need extra help with either they arent getting the material intellectually or dont have good habits and arent practicing as such. I would like to teach them an extra day, preferably sunday, to which I wouldnt get paid on this day nor would it be added up towards a lesson.

Is it okay to tutor (aka teach)? I need a learned person to please tell me if this is sinful. Preferably if this is traditionally sinful. Because I if possible, I like commit no sin, since tutoring is quite taxing on the mind. Because in every lesson and every student, you give a tiny slice of yourself...if the teacher has fully good intentions that is. Thank you :)