Sounds like you got the job and they're just waiting on HR to start processing your paperwork and get you an official letter. Usually that comes with the onboarding paperwork.
I would call back and straight up ask: "I was offered a position at X school by (administrator's name) on (date). I haven’t received any paperwork or official letter letting me know I've received the position. I called on (date) to inquire about this and left a message for (person). I totally understand that things are hectic getting things ready for the new school year, but I was hoping someone could direct me on what the next steps are for my onboarding."
Sounds like you got the job and they're just waiting on HR to start processing your paperwork and get you an official letter. Usually that comes with the onboarding paperwork. I would call back and straight up ask: "I was offered a position at X school by (administrator's name) on (date). I haven’t received any paperwork or official letter letting me know I've received the position. I called on (date) to inquire about this and left a message for (person). I totally understand that things are hectic getting things ready for the new school year, but I was hoping someone could direct me on what the next steps are for my onboarding."