You CAN drop him from your life when you turn 18, but that doesn’t mean schools are going to dismiss him as easily. Depending on the state, your father’s obligations for child support may end at 18 or may continue to 21. Your mother may want him to be obligated to continue paying.
It’s a difficult situation but not uncommon. Many students have parents who make a middle class or higher income who won’t or can’t pay for college. Colleges still look to those parents to pay.