It depends on what you mean by "rights". An 18 year old is an adult under the law who cannot buy alcohol but can smoke and die for his country. The 18 year old makes his own legal choices, and can deny his parents access to medical information about him/her. Parents can claim the 18 year old as a dependent on their taxes so long as they provide 51% of the support of the "new adult". When a child reaches the age of 18 he/she is now an adult and the parents really don't have "rights" as they did when the child was under 18 years of age. The fact of being or not being in high school is not critical to this determination of parental rights.
It depends because you are still in high school and once you turn 18, you acquire new rights that are associated with being independent. You also become responsible with new responsibilities and are held personally answerable for your actions. You can get more information here http://opi.mt.gov/pdf/LegalDivision/Now18.pdf.