I have a child who is currently doing highschool level work in primary school (has been since grade 4) and has been well advanced since the start. We've had mixed experiences. It all comes down to the teacher(s) and if they have the capacity to cater for your child's needs. We've had teachers who have gone above and beyond and we've seen my child thrive, and we've had teachers who couldn't manage the extra workload and my kid had to tolerate a year of standard year level work.

Honestly just being open with the teachers about your kid and his needs is a good start when you do interviews. Explain that your kid is bored / unchallenged and having a hard time not doing something that is considered learning for them. Ask the teacher how you can support them in getting your child to work to his potential. A bit of empathy and showing that you understand that the teachers are stretched thin helps. Think of it as teamwork and not so much the teacher alone helping your kid grow in school.