When you want too or need to transfer from one high school to another high school you'll need to speak with your current school staff as well as the new school staff to enroll in the new school.
Then at the old school you'll usually fill out some paperwork for the transfer which then you will take to the new school so that your courses and credits transfer over to the new high school.
You'll then also fill out some new paperwork with your details etc at the new high school and then the school will take care of the rest at getting your information transferred over to the new school from the old high school.
Transferring from one high school to a new high school is not that complicated although it is a headache and you should make sure that it's worth the time and paperwork as well as your loss of your grades when you transfer to a new high school.
When you transfer to a new high school your grades are not transferred over however your courses are.