What is the name of your state (only U.S. law)? I live in Las Vegas Nevada and I am 5 days away from being 18. I may switch at the semester but my counselor said my mother has to take me out of school. If at the age of 18, your parents are not responsible for you, isn't it possible for me to unenroll myself as long as I am enrolling into another school?
Check with the school. You may have to fill out a form that indicates you are completely responsible for yourself and to take your parents out of the loop.
If you want to enroll in another school, check with BOTH schools to see what they need. They will almost certainly need a proof of residency (in the new school's area) and may require some documentation that you are now responsible for your own affairs.
Are you prepared to take this step? Age 18 means you can make some decisions, to be certain, but it also means that you can be left on your own with only the clothes on your back. if you are prepared for that possibility, by all means, ignore your parents' desires. But, be careful as this may not be a step you can un-do should it blow up in your face.
Why do you want to move to another school so close to graduation?