The first NTR event between the two is the week after the first weekly exam. She'll pay her debt to him somehow. In order for her to offer herself, she needs a high corruption and to have scored low on the exam. MC can then convince her to do it in a preceding event.
For the most recent NTR event between the two, call Soushi to the Office to decide how the deal over having Natsuha (Ayumi's ex) as a student will go down. If Ayumi has a high enough corruption (something like 30 irc), MC can offer Ayumi then convince her afterwards. Then just wait until the event happens.