All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If the school conditions that a student should not make use of books or get help from other people in these tests, then it is not permissible for you to do so, because this is cheating and it is forbidden to cheat as clarified in Fatwa 81709, so please refer to it.
The fact that the school is not strict on supervision does not justify cheating, and hence you should repent to Allaah and inform the school about what you did in order to put your mind at ease.
Nonetheless, if the school permits this, and allows the students to check the answers by any means, then there is no harm in doing so, and you are not sinful in this case, but this is very unlikely.
Allaah Knows best.