Complex sentence: Wikipedia welcomes new editors, and having assignments for student editors can be a great way to add valuable content and to show new users how to edit, but sometimes volunteer editors burdened with cleaning up a mess that contradicts Wikipedia's norms (known as).
Split into simpler sentences: Wikipedia welcomes new editors, and having assignments for student editors can be a great way to add valuable content and to show new users how to edit. However, some assignments result in a net negative for Wikipedia, with volunteer editors burdened with cleaning up a mess created by edits that are contrary to Wikipedia's norms (known as).