Note your other schools are ones parents have to pay a tuition fee for. That means they are probably from more financially well off environments, where isolation was probably easier and more effective. The schools are also self selecting, so can pick students more likely to be less exposed to covid.
Public schools have to take everyone, including those who parents are probably essential (read-expendable) workers and who may live in tighter, more crowded circumstances. The kids might not be as vulnerable to infection, but they can still be carriers. Teachers are adults who might be living with immuno compromised relatives. They should be raised in priority for vaccination and proper PPE should be ensured and issued.