The problem as you mentioned before is ongoing infection from asymptomatic carriers, or symptomatic carriers suppressing their symptoms, or confirmed cases lying about contact tracing.
Am going to lay some assumptions here:
March school holidays onwards, folks had been talking about parents or staff who may lie about their travel history when they return- possibility of staff-to-staff, or staff-to-student, or student-to-student is ever present;
coupled with those covidiots who slipped out during their SHN/LOA period;
as well as those asymptomatic carriers totally unaware that they are spreading the virus around.
given two cycles of spreading and incubation, these factors were enough breeding ground to form the reservoir Pinkie was talking about currently.