I would imagine that each school has a variety of options, all of which must be approved of by parents/guardians. (My kid is NOT at Andover.)
For example, I had to complete a variety of blanket permission forms for my child’s school - whether they could use a taxi, whether other parents or relatives could transport them. Info for settling them up to use the T.
One point of disagreement with me and my child is that I refused to give blanket permission for them to ride in a fellow student’s car, whether or not the driver was over 18.
Of course, since boarders have not been able to casually sign out this year or keep cars on campus, the whole issue was moot.