schema报错

相关代码：

<?xml version="1.0" encoding="utf-8" ?>
<!DOCTYPE teaching-plan SYSTEM "plan.dtd">
<!--教学计划-->
<teaching-plan xmlns="http://www.imooc.com/plan-schema"
               xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
               xsi:schemaLocation="http://www.imooc.com/plan-schema  plan.xsd">
    <course id="3309">
        <course-name>大学英语</course-name>
        <class-hour>36</class-hour>
        <exam-form>考试</exam-form>
     </course>
    <course id="3310">
        <course-name>高等数学</course-name>
        <class-hour>70</class-hour>
        <exam-form>考试</exam-form>
    </course>
    <course id="3311">
        <course-name>计算机应用基础</course-name>
        <class-hour>108</class-hour>
        <exam-form>上机考试</exam-form>
    </course>
</teaching-plan>

相关代码：

<?xml version="1.0" encoding="UTF-8" ?>
<schema xmlns="http://www.w3.org/2001/XMLSchema"
    targetNamespace="http://www.imooc.com/plan-schema" elementFormDefault="qualified">
    <element name="teaching-plan">
        <complexType>
            <sequence>
                <element name="course" minOccurs="1" maxOccurs="100">
                    <complexType>
                        <sequence>
                            <element name="course-name" type="string"></element>
                            <element name="class-hour" >
                                <simpleType>
                                    <restriction base="integer">
                                        <minInclusive value="20"></minInclusive>
                                        <maxInclusive value="110"></maxInclusive>
                                    </restriction>
                                </simpleType>
                            </element>
                            <element name="exam-form" type="string"></element>
                        </sequence>
                        <attribute name="id" type="string" use="required"></attribute>
                    </complexType>
                </element>
            </sequence>
        </complexType>
    </element>
</schema>

问题描述：

id,exam-form字段下面红线报错

相关截图：