Am I sinful for not informing the employees about the new law that restricts book sales to undergraduate students only, and for buying a book designated for graduate studies at a university-subsidized price?

It appears that there is no sin upon you for not informing your colleagues about what your teacher told you regarding the new law, because the mere notification from the teacher does not establish certainty about the law's issuance and applicability. It is also possible that the electronic system's rejection of the transaction was due to a malfunction. With these possibilities, the default remains that purchasing the books is permissible. However, if you become certain that the purchase transaction occurred by mistake and that you are not entitled to buy them at the discounted price, then you must refer back to the employees to correct the transaction.