Is there a way to set Assignment Group Weights globally as an admin for everyone at once?
Kristy, there is no easy way to do this on the front end of Canvas. The only way I can think of is using the Canvas API. If you're not familiar with the API, here is a good place to start - Canvas APIs: Getting started, the practical ins and outs, gotchas, tips, and tricks
PS - one other thing you could try is reviewing the API information and then posting your question in the Canvas Developers area of the Community. They are the ones who do this type of Canvas programming.
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