Is taking students' pens and books, and the school books she used for teaching, considered an act of embezzlement (ghulul) on the part of the teacher, and what is incumbent upon her to do with these materials after two or three years have passed since she left her job?

Ghulul is taking from the spoils of war secretly before their distribution, or absolute betrayal. Therefore, your forgetting of others' books and pens is not considered ghulul. There is no sin in taking books that students have discarded, and you are not obliged to return them. However, you must return the books that the school gave you, otherwise, that would be a form of ghulul in the general sense (betrayal of trust). As for students' pens, if they gifted them to you, they are yours. If they lent them to you, you must return them or give charity equivalent to their value if returning them is not possible.