The simple definition is "greetings," but you may encounter aisatsu in many different contexts in school, including but not limited to:
a) Standing at the school gate and saying "ohayou gozaimasu" and "good morning" to staff and students arriving at school. Alternatively, walking around school in between periods or visiting clubs and saying "hi."
b) Announcements given by kouchou-sensei or kyoutou-sensei, or any other staff member, during morning meeting in the staff room.
c) Speeches given during a major ceremony.
d) Saying "osaki ni shitsurei shimasu" or "otsukaresama desu" to fellow teachers at the end of the school day as you all leave for home.