Is it a sin for a student to refuse to give her classmate the lecture notes she wrote herself, and is she entitled to do so, knowing that these notes are essential for the exam?

The papers written from following lectures are the private property of their owner, and she has full freedom to donate them or prohibit copying from them. There is no sin in not giving them to colleagues or preventing them from copying from them. It is better to cooperate in righteousness and piety and help those who can be helped, unless there is deception involved.