_{New, detailed answer and explanation to an old, frequently asked question…}

Short answer: If you don’t add elementFormDefault="qualified" to xsd:schema, then the default unqualified value means that locally declared elements are in no namespace.

There’s a lot of confusion regarding what elementFormDefault does, but this can be quickly clarified with a short example…

Streamlined version of your XSD:

<?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
        xmlns:target="http://www.levijackson.net/web340/ns"
        targetNamespace="http://www.levijackson.net/web340/ns">
  <element name="assignments">
    <complexType>
      <sequence>
        <element name="assignment" type="target:assignmentInfo" 
                 minOccurs="1" maxOccurs="unbounded"/>
      </sequence>
    </complexType>
  </element>
  <complexType name="assignmentInfo">
    <sequence>
      <element name="name" type="string"/>
    </sequence>
    <attribute name="id" type="string" use="required"/>
  </complexType>
</schema>

Key points:

• The assignment element is locally defined.
• Elements locally defined in XSD are in no namespace by default.
  • This is because the default value for elementFormDefault is unqualified.
  • This arguably is a design mistake by the creators of XSD.
  • Standard practice is to always use elementFormDefault="qualified"
    so that assignment is in the target namespace as one would
    expect.
• It is a rarely used form attribute on xs:element declarations for which elementFormDefault establishes default values.

Seemingly Valid XML

This XML looks like it should be valid according to the above XSD:

<assignments xmlns="http://www.levijackson.net/web340/ns"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://www.levijackson.net/web340/ns try.xsd">
  <assignment id="a1">
    <name>John</name>
  </assignment>
</assignments>

Notice:

• The default namespace on assignments places assignments and all of its descendents in the default namespace (http://www.levijackson.net/web340/ns).

Perplexing Validation Error

Despite looking valid, the above XML yields the following confusing validation error:

[Error] try.xml:4:23: cvc-complex-type.2.4.a: Invalid content was
found starting with element ‘assignment’. One of ‘{assignment}’ is
expected.

Notes:

• You would not be the first developer to curse this diagnostic that seems to say that the content is invalid because it expected to find an assignment element but it actually found an assignment element. (WTF)
• What this really means: The { and } around assignment means that validation was expecting assignment in no namespace here. Unfortunately, when it says that it found an assignment element, it doesn’t mention that it found it in a default namespace which differs from no namespace.

Solution

• Vast majority of the time: Add elementFormDefault="qualified" to the xsd:schema element of the XSD. This means valid XML must place elements in the target namespace when locally declared in the XSD; otherwise, valid XML must place locally declared elements in no namespace.
• Tiny minority of the time: Change the XML to comply with the XSD’s
  requirement that assignment be in no namespace. This can be achieved,
  for example, by adding xmlns="" to the assignment element.

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_{Credits: Thanks to Michael Kay for helpful feedback on this answer.}