Live your life and the rest will work out however it is meant to work out.
Hopefully, you have decades to be concerned with planning your life with (not "for" or "around") a man. This high school boyfriend may be that man but there is 98% chance that he will not be that man. Focus on school, sorority, and career and then you will eventually be equipped to focus on a family (beyond the family that you currently have).
That does not mean that you have to emotionally distance yourself or break up with him while he is still in high school.
Simply do not let this relationship dictate your school and sorority plans. It will be supercute and LMN-worthy if (read: IF) he attends the same college next year, joins a fraternity, and you two are still together. One or all of those may not happen so keep doing what you have to do to never be one of those people who says "I wish I had...."