I want to leave my child's father when my youngest turns 18 However he will still be in high school

Leaving your child's father would have nothing to do with "abandonment". You may still have to pay child support, however; it depends on the law of whichever state or country you are in.

Child abandonment is generally a criminal charge referring to leaving a minor (usually very young) child without adult supervision. So I cannot see you being charged with child abandonment. In a very few states, failure to support would also be considered child abandonment, but in those cases a support order needs to be first issued by a court.