Yes, you can have as many memberships as you need for your students. Keep in mind that they may expire at different times since they expire one year after purchase.

The only way to transfer a membership from one student to another is to pass the student account of the old student to a student account of a new student. Note that you cannot transfer the membership to an existing student. To transfer the account, go to your school's student list, if you're a teacher affiliated with a school who assigns you licenses, or go into your classroom. Find the old student from the list and edit the student, giving the student a new name, username and password. Note that any data from apps used by the old student will have to be deleted by you (this only applies to apps that collect data). To do this, select the button under the app, find the student, select "Show Progress" and find the "Clear Data" button(s).

Yes, you have 15 days to return a license. To do this, go to your Order History and select "start a return".