If we the parents leave the USA but our then 18 year old wants to stay with her green card, can she?

Generally, a Lawful Permanent Resident (holder of a "Green Card") is lawfully authorized to remain in the U.S. (to go to college or for other reasons), notwithstanding whether her parents may depart. This is true regardless of whether the parents may intentionally (through renouncing their status) or unintentionally (through remaining outside the U.S. for an extensive time) terminate their Permanent Resident status. Additionally, depending upon when she became a Permanent Resident, the Lawful Permanent Resident daughter may become eligible to apply to become a naturalized U.S. citizen when she reaches age 18. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.