diff --git a/.cospec/plan/changes/fix-admin-multi-issues/task.md b/.cospec/plan/changes/fix-admin-multi-issues/task.md
index 4ff78dd..fcbae3b 100644
--- a/.cospec/plan/changes/fix-admin-multi-issues/task.md
+++ b/.cospec/plan/changes/fix-admin-multi-issues/task.md
@@ -202,3 +202,23 @@
         - 在 `clearAuth()` 调用后、路径检查之前，添加 `fetch('/api/clear_session.php', { method: 'POST', headers: { 'Content-Type': 'application/json' } })` 调用
         - 使用 try-catch 包裹，失败时仅输出 console.warn 警告，不阻塞后续重定向逻辑
         - `/api/clear_session.php` 是同源路径（由 Nginx 直接处理，不经过后端 FastAPI），不需要 Authorization header，也不会触发跨域问题
+
+### 阶段 9：修复 500 响应 CORS 头丢失 + admin.js 变量重复声明
+
+- [x] 9.1 修复 500 响应 CORS 头丢失
+     【目标对象】`backend/middleware/auth_middleware.py`
+     【修改目的】当路由层抛出异常导致 500 时，BaseHTTPMiddleware 的 call_next 返回的 500 响应不会经过 CORSMiddleware，导致 CORS 头缺失。浏览器报告"CORS Missing Allow Origin"，前端无法读取错误信息。
+     【修改方式】在 dispatch 方法中将 `return await call_next(request)` 改为 try-except 包裹，确保所有响应都有 CORS 头
+     【修改内容】
+        - 用 try-except 包裹 `call_next(request)` 调用
+        - 在 try 中获取 response 后检查是否已有 CORS 头，若无则补充
+        - 在 except 中捕获路由层异常，返回带 CORS 头的 500 响应
+
+- [x] 9.2 修复 admin.js selectedStudentIds 重复声明
+     【目标对象】`frontend/assets/js/admin.js`、`frontend/admin/conduct.php`、`frontend/admin/homework.php`
+     【修改目的】admin.js 通过 footer.php 在所有 admin 页面加载，其 `let selectedStudentIds = []` 与 conduct.php 和 homework.php 页面级 `<script>` 中的同名声明冲突，导致 SyntaxError
+     【修改方式】将 `let` 改为 `var`，`var` 允许重复声明
+     【修改内容】
+        - admin.js 第12-16行：`let` → `var`（4个变量声明）
+        - conduct.php 第59行：`let selectedStudentIds = []` → `var selectedStudentIds = []`
+        - homework.php 第94行：`let selectedStudentIds = []` → `var selectedStudentIds = []`
diff --git a/backend/middleware/auth_middleware.py b/backend/middleware/auth_middleware.py
index 80c6ecf..34dca08 100644
--- a/backend/middleware/auth_middleware.py
+++ b/backend/middleware/auth_middleware.py
@@ -107,7 +107,18 @@ class AuthMiddleware(BaseHTTPMiddleware):
             logger.error(f"认证中间件异常: {e}", exc_info=True)
             return self._cors_response(request, 401, "认证服务异常，请稍后重试")
         
-        return await call_next(request)
+        try:
+            response = await call_next(request)
+            # 为所有响应确保CORS头存在（防止路由层异常导致CORS头丢失）
+            origin = request.headers.get("origin", "")
+            allowed_origins = settings.CORS_ORIGINS or []
+            if origin in allowed_origins and not response.headers.get("access-control-allow-origin"):
+                response.headers["access-control-allow-origin"] = origin
+                response.headers["access-control-allow-credentials"] = "true"
+            return response
+        except Exception as e:
+            logger.error(f"[Auth] call_next异常: {e}", exc_info=True)
+            return self._cors_response(request, 500, "服务器内部错误")
     
     def _cors_response(self, request: Request, status_code: int, message: str) -> JSONResponse:
         """创建带CORS头的响应"""
diff --git a/frontend/admin/conduct.php b/frontend/admin/conduct.php
index f0d9b8e..11b6eeb 100644
--- a/frontend/admin/conduct.php
+++ b/frontend/admin/conduct.php
@@ -56,7 +56,7 @@ include __DIR__ . '/../includes/header.php';
 </div>
 
 <script>
-let selectedStudentIds = [];
+var selectedStudentIds = [];
 
 async function loadStudents() {
     const res = await apiGet('/api/admin/students');
diff --git a/frontend/admin/homework.php b/frontend/admin/homework.php
index d10e31c..2e3904f 100644
--- a/frontend/admin/homework.php
+++ b/frontend/admin/homework.php
@@ -91,7 +91,7 @@ include __DIR__ . '/../includes/header.php';
 </div>
 
 <script>
-let selectedStudentIds = [];
+var selectedStudentIds = [];
 
 // 初始化扣分配置
 const hwMaxPoints = window.HOMEWORK_MAX_POINTS || 3;
diff --git a/frontend/assets/js/admin.js b/frontend/assets/js/admin.js
index d96f58f..86873ca 100644
--- a/frontend/assets/js/admin.js
+++ b/frontend/assets/js/admin.js
@@ -9,11 +9,11 @@
  * 版权所有 © Sea Network Technology Studio
  */
 
-// 全局变量
-let selectedStudentIds = [];
-let currentPage = 1;
-let totalPages = 1;
-let currentHistoryPage = 1;
+// 全局变量（使用 var 避免与页面级 let 重复声明冲突）
+var selectedStudentIds = [];
+var currentPage = 1;
+var totalPages = 1;
+var currentHistoryPage = 1;
 
 // 显示批量加减分模态框
 function showBatchPointsModal() {