XML Schema How to Restrict Attribute by minLength \ maxLength

I've been trying to solve this issue for a few hours now, the problem is that my validation process doesn't really take into account anything I type inside the restriction.

This is my XSD:

<xs:schema attributeFormDefault="unqualified" elementFormDefault="qualified" xmlns:xs="http://ift.tt/tphNwY">
<xs:element name="Schools-xml">
<xs:complexType>
  <xs:sequence>
    <xs:element name="School-xml" maxOccurs="unbounded" minOccurs="0">
      <xs:complexType>
        <xs:sequence>
          <xs:element type="xs:string" name="Id"/>
          <xs:element name="Preferences">
            <xs:simpleType>
              <xs:restriction base="xs:string">
                <xs:minLength value="5"/>
                <xs:maxLength value="5"/>
              </xs:restriction>
            </xs:simpleType>
          </xs:element>
        </xs:sequence>
      </xs:complexType>
    </xs:element>
  </xs:sequence>
</xs:complexType>

Xml Sample:

<?xml version="1.0" encoding="utf-8"?>
<Students_With_Preferences-xml xmlns="http://ift.tt/1GG1t2W">                                 
<Student-xml>
    <District>b</District>                              
    <Preferences>(sc1,20)(sc2,15)</Preferences>     
</Student-xml>
<Student-xml>
    <District>d</District>
    <Preferences>(sc2,15)|(sc1,10)</Preferences>    
</Student-xml>

As you can see only element with length 5 is allowed, but when I run my code it allows any length. Just to be clear, everything else is validate properly, just this annoying error.

I tried to fix this issue using the regex option but as u guessed its not working as well.

Validation Code(C#):

    private Boolean ValidateXML(HttpPostedFileBase file, string xsd_path)
    {
        bool errors = false;
        string filename = "";
        XmlSchemaSet schemas = new XmlSchemaSet();
        XmlDocument xml_doc = new XmlDocument();
        try
        {
            filename = file.FileName;
            schemas.Add("", xsd_path);
            xml_doc.Load(file.InputStream);
            foreach (XmlNode node in xml_doc.ChildNodes.Item(1).ChildNodes)
            {
                XDocument doc = XDocument.Parse(node.OuterXml);
                doc.Validate(schemas, (o, e) =>
                {
                    ViewBag.msg = "Error validating - " + filename + " - " + e.Message;
                    errors = true;
                });
                if (errors) break;
            }
        }
        catch (Exception ex)
        {
            errors = true;
            string msg = "Error validating - " + filename + " - " + ex.Message;
            throw new Exception(msg);
        }
        if (!errors)
        {
            ViewBag.msg = filename + " was validated succesfully";
        }

        return !errors;
    }

I would really appreciate any help on how to fix this issue, Thanks!