Here are a few options, though none are particularly common:
* Bench: This is a common term for the group of judges presiding over a court, which could include magistrates.
* Panel: This is a more general term for any group of people working together, which could be applied to a group of magistrates.
* Assembly: This is a more formal term, suggesting a gathering of magistrates for a specific purpose.
Ultimately, the best collective noun to use depends on the specific context.