Using .NET to validate XML against a schema
We can't see your code, but In many implementations this is handled by redirecting the request for the .xsd to the local copy using a catalog resolver. There is a property XmlReaderSettings.XmlResolver that can be used for this. See XMLCatalog.net for an Apache-licensed implementation you can use.
A side-effect of this is that you can keep all schemas cached locally. This is especially important since W3C will block excessive reads to their site and randomly your code (or worse, your customer's code) will begin to fail.