XML 问题： must refer to an existing element. 关于前缀

刚写了这样一个XML，始终不是valid。
捣鼓半天，把所有的xs:加上了，结果还真通过了，这个是什么原因呢？
报错信息是 '学生' must refer to an existing element.

 <?xml version="1.0" encoding="UTF-8"?>
<schema xmlns="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">

    <element name="姓名" type="string"/>

    <element name="性别">
        <simpleType>
            <restriction base="string">
                <pattern value="(男|女)"/>
            </restriction>
        </simpleType>
    </element>
    <element name="年龄">
        <simpleType>
            <restriction base="integer">
                <minExclusive value="0"/>
                <maxExclusive value="150"/>
            </restriction>
        </simpleType>
    </element>

    <attribute name="学号" type="integer"/>

    <complexType name="stu">
        <sequence maxOccurs="1">
            <element ref="姓名"/>
            <element ref="性别"/>
            <element ref="年龄"/>
        </sequence>
        <attribute ref="学号" use="required"/>
    </complexType>


    <element name="学生" type="stu"/>

        <complexType name="myT">
        <sequence>
            <element ref="学生" maxOccurs="unbounded"/>
        </sequence>     
    </complexType>




        <element name="学生名册" type="myT"/>

这个是后来加上的，没问题

 <?xml version="1.0" encoding="UTF-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" attributeFormDefault="unqualified">
    <xs:element  name="学生名册" type="myT"/>
    <xs:complexType name="myT">
        <xs:sequence maxOccurs="unbounded">
            <xs:element ref="学生"/>
        </xs:sequence>
    </xs:complexType>

    <xs:element name="学生" type="stu"/>

    <xs:element name="姓名" type="xs:string"/>

    <xs:element name="性别">
        <xs:simpleType>
            <xs:restriction base="xs:string">
                <xs:pattern value="(男|女)"/>
            </xs:restriction>
        </xs:simpleType>
    </xs:element>
    <xs:element name="年龄">
        <xs:simpleType>
            <xs:restriction base="xs:integer">
                <xs:minInclusive value="0"/>
                <xs:maxExclusive value="150"/>
            </xs:restriction>
        </xs:simpleType>
    </xs:element>
    <xs:attribute name="学号" type="xs:integer"/>

    <xs:complexType name="stu">
        <xs:sequence maxOccurs="1">
            <xs:element ref="姓名"/>
            <xs:element ref="性别"/>
            <xs:element ref="年龄"/>
        </xs:sequence>
        <xs:attribute ref="学号" use="required"/>
    </xs:complexType>




</xs:schema>

http://forum.spring.io/forum/spring-projects/boot/724165-repackager-source-must-refer-to-an-existing-file