Here are some options you could use, depending on the context:
Informal:
* A pack of rowdies
* A mob of rowdies
* A bunch of rowdies
* A crew of rowdies
* A gaggle of rowdies
More descriptive:
* A riotous assembly
* A boisterous throng
* A boisterous crowd
* A raucous band
Ultimately, the best collective noun depends on the specific situation and tone you're aiming for.