Sketches that you upload are seperate already as there are no "default" sketches except in the examples from libraries.
As you are uploading your own set your students should only see those sketches.
If you are talking about the default "example sketches included with libraries then this is a slightly different matter.
Currently you cannot hide the default libs although that would be a useful feature on this occasion.
However as you have uploaded extra libs with your grove kits your students should simply select "CUSTOM" in the libs selection which would only show items you have placed there.
Am also attaching a help file (PDF) that may be of use to you for importing sketches and libs.
Bob.