The law varies from state to state.
In some states you may legally be able to sign yourself out of school and drop out of school without parent consent.
However in some states such as Kansas you can drop out of school at 18 but require consent from your parents or a court to legally be able to drop out of school.
Kansas law allows students to drop out of school before they graduate and before they turn 18 if they are age 16 or 17 and they have their parents, guardians, or a court give written consent to their withdrawal.
However if we're talking about signing yourself out of school for the day or for a doctors appointment etc to leave school temporarily then yes in most places you can sign yourself out of school for the day to attend those appointments.
Some schools also have what are called personal days where you can take off for the day of school without parent consent.
Then on those days you can do whatever you want too.
Although once you get to 18 years of age you're almost ready to Graduate and no longer need to or have to attend regular school.
However if you want to attend College after highschool then you can do so and leave college at any time you want too.
However for regular school even though you're 18 and legally an adult the school is still mandatory for you to attend and the school has rules that you must follow.