fix(teacher): 修复年级切换时班级值未重置的问题

移除年级切换时对班级值的重置操作，保留课程值的重置

@@ -2,7 +2,7 @@
  * @Author: hookehuyr hookehuyr@gmail.com
  * @Date: 2025-01-20 10:00:00
  * @LastEditors: hookehuyr hookehuyr@gmail.com
- * @LastEditTime: 2025-06-27 14:46:06
+ * @LastEditTime: 2025-06-27 15:29:06
  * @FilePath: /mlaj/src/views/teacher/myClassPage.vue
  * @Description: 我的班级页面
 -->
@@ -30,7 +30,7 @@
       <!-- 筛选器 -->
       <div>
         <van-sticky>
-          <van-dropdown-menu active-color="#10b981" swipe-threshold="3">
+          <van-dropdown-menu active-color="#10b981" swipe-threshold="2">
             <van-dropdown-item v-model="selectGradeValue" :options="gradeOption" @change="handleGradeChange" />
             <van-dropdown-item v-model="selectClassValue" :options="classOption" @change="handleClassChange" />
             <van-dropdown-item v-model="selectCourseValue" :options="courseOption" @change="handleCourseChange" />
@@ -239,7 +239,6 @@ const handleGradeChange = async (val) => {
   console.log('val', val);
   selectGradeValue.value = val;
   // 重置班级和课程选择
-  selectClassValue.value = null;
   selectCourseValue.value = null;
 
   // 根据年级ID 更新过滤列表