In Mississippi can your parents force you to remain in the family home until you reach 21 if you haven't finished high school?

No. The age for compulsory education in Mississipi is 17. The basic reason for the age of majority to be set at 21 pertains to matters child support orders, care of a physical or mentally challenged person and in some situations the legal liability of the parents or legal guardian. The absence of such issues is what allows a person who is at least 18 to receive an age waiver from the court.