Ciao a tutti ho qualche problema con la validazione tramite schema allego il codice ed i due file xml ed xsd ,ottengo lo stesso errore anche usando file tratti da esempi su internet quindi penso che l'errore sia proprio mio...
il codice 
try {
                XmlReaderSettings set = new XmlReaderSettings();
                set.ValidationType = ValidationType.Schema;
                XmlSchemaSet s = new XmlSchemaSet();
                XmlSchema schema = new XmlSchema();
                schema.SourceUri = @"C:\Users\gianmarco\documents\visual studio 2015\Projects\BookSamples\BookSamples\XMLFile1.xsd";
                s.Add(schema);
                set.Schemas = s;               
                string path = Server.MapPath("XMLEmployees012345.xml");
                XmlReader r = XmlReader.Create(path, set);
                while (r.Read());
                r.Close();
            } catch(Exception ex)
            {
}
il file xml 
<Employees>
  <Employee><ID>0</ID><First>Primo</First><Last>Segundo</Last><Salary>2500</Salary></Employee>
  <Employee><ID>1</ID><First>Primo</First><Last>Segundo</Last><Salary>2500</Salary></Employee>
  <Employee><ID>2</ID><First>Primo</First><Last>Segundo</Last><Salary>2500</Salary></Employee>
  <Employee><ID>3</ID><First>Primo</First><Last>Segundo</Last><Salary>2500</Salary></Employee>
  <Employee><ID>4</ID><First>Primo</First><Last>Segundo</Last><Salary>2500</Salary></Employee>
</Employees>
il file xsd 
<?xml version="1.0" ?>
<xs:schema id="Employees" xmlns:xs="
http://www.w3.org/2001/XMLSchem">
  <xs:element name="Employees" >
    <xs:complexType>
      <xs:sequence maxOccurs="unbounded">
        <xs:element name="Employee" type="Employee" />
      </xs:sequence>
    </xs:complexType>
  </xs:element>
    <xs:complexType name="Employee">
      <xs:sequence maxOccurs="unbounded">
        <xs:element name="ID" type="xs:string" />
        <xs:element name="First" type="xs:string" />
        <xs:element name="Last" type="xs:string" />
        <xs:element name="Salary" type="xs:string" />
      </xs:sequence>      
    </xs:complexType>
</xs:schema>
l'errore :
Elemento 'Employees' non dichiarato.
ho usato i due file  xml ed xsd qua 
http://www.w3schools.com/xml/schema_complex_indicators.asp ed ho ottenuto lo stesso errore,
grazie in anticipo