Is it obligatory to stop working at an institute that photocopies books for students without the publisher's permission, knowing the difficulty of obtaining original books and their high price?

You are not obligated to leave your work at the aforementioned institute, nor are you required to investigate and ascertain whether the copies printed by the institute are authorized for duplication. The default assumption is permissibility. Even if you were to confirm that the institute copies books without the permission of their owners, this does not make your work in teaching unlawful. However, you should advise them and prohibit them from infringing upon the rights of others.