diff --git a/account/views/admin.py b/account/views/admin.py
index 5a4be72..67b47b6 100644
--- a/account/views/admin.py
+++ b/account/views/admin.py
@@ -22,17 +22,12 @@ from ..serializers import (
 )
 
 
-# ks251XXX 或者 ks2510XX 返回 251 或者 2510
-# 其他返回 None
+# ks251XXX 或者 ks2510XX 返回 251 或者 2510，其他返回 None。
+# 班级号限定 3~4 位，与前端 ButtonWithSearch 的 /^ks\d{3,4}/ 保持一致；
+# 原来的 \d+ 会贪婪吃掉后面的数字（ks251001 会返回 251001 而不是 251）。
 def get_class_name(username):
-    if username.startswith("ks"):
-        result = re.search(r"ks\d+", username)
-        if result:
-            return result.group(0)[2:]
-        else:
-            return None
-    else:
-        return None
+    result = re.match(r"ks(\d{3,4})", username)
+    return result.group(1) if result else None
 
 
 class UserAdminAPI(APIView):
diff --git a/conf/views.py b/conf/views.py
index dddce53..bc812fa 100644
--- a/conf/views.py
+++ b/conf/views.py
@@ -19,7 +19,7 @@ from problem.models import Problem
 from submission.models import Submission
 from utils.api import APIView, AsyncAPIView, CSRFExemptAPIView, validate_serializer
 from utils.cache import JsonDataLoader
-from utils.shortcuts import get_env
+from utils.shortcuts import get_env, strip_class_prefix
 from utils.websocket import push_config_update
 from utils.xss_filter import XSSHtml
 
@@ -239,8 +239,5 @@ class ClassUsernamesAPI(AsyncAPIView):
         classroom = request.GET.get("classroom", "")
         if not classroom:
             return self.error("需要班级号")
-        prefix = f"ks{classroom}"
-        names = [
-            user.username[len(prefix) :] if user.username.startswith(prefix) else user.username async for user in User.objects.filter(class_name=classroom).order_by("-create_time")
-        ]
+        names = [strip_class_prefix(user.username, classroom) async for user in User.objects.filter(class_name=classroom).order_by("-create_time")]
         return self.success(names)
diff --git a/flowchart/views/admin.py b/flowchart/views/admin.py
index d0d1880..1f2f101 100644
--- a/flowchart/views/admin.py
+++ b/flowchart/views/admin.py
@@ -8,6 +8,7 @@ from account.decorators import teacher_admin_required
 from account.models import AdminType, User
 from problem.models import Problem
 from utils.api import APIView
+from utils.shortcuts import strip_class_prefix
 
 from ..models import FlowchartSubmission, FlowchartSubmissionStatus
 
@@ -64,12 +65,6 @@ for _w in CUSTOM_WORDS:
     jieba.add_word(_w, freq=9999)
 
 
-def get_real_name(username, class_name):
-    if class_name and username.startswith("ks"):
-        return username[len(f"ks{class_name}") :]
-    return username
-
-
 class FlowchartStatisticsAPI(APIView):
     @teacher_admin_required
     def get(self, request):
@@ -169,7 +164,7 @@ class FlowchartStatisticsAPI(APIView):
         if all_users_dict:
             for uname in set(all_users_dict.keys()) - submitted_users:
                 class_name = all_users_dict[uname]
-                real_name = get_real_name(uname, class_name)
+                real_name = strip_class_prefix(uname, class_name)
                 unaccepted.append({"username": uname, "real_name": real_name})
 
         # 5. Word cloud from feedback + suggestions + criteria comments
diff --git a/submission/views/admin.py b/submission/views/admin.py
index 928ed93..35acfb0 100644
--- a/submission/views/admin.py
+++ b/submission/views/admin.py
@@ -5,16 +5,11 @@ from account.models import AdminType, User
 from judge.tasks import judge_task
 from problem.models import Problem
 from utils.api import APIView
+from utils.shortcuts import strip_class_prefix
 
 from ..models import JudgeStatus, Submission
 
 
-def get_real_name(username, class_name):
-    if class_name and username.startswith("ks"):
-        return username[len(f"ks{class_name}") :]
-    return username
-
-
 class SubmissionRejudgeAPI(APIView):
     @super_admin_required
     def get(self, request):
@@ -132,7 +127,7 @@ class SubmissionStatisticsAPI(APIView):
             unaccepted_usernames = set(all_users_dict.keys()) - submitted_usernames
             for username in unaccepted_usernames:
                 class_name = all_users_dict[username]
-                real_name = get_real_name(username, class_name)
+                real_name = strip_class_prefix(username, class_name)
                 unaccepted.append({"username": username, "real_name": real_name})
 
         # 计算人数完成率
diff --git a/utils/shortcuts.py b/utils/shortcuts.py
index 09b2af6..d3f3850 100644
--- a/utils/shortcuts.py
+++ b/utils/shortcuts.py
@@ -36,6 +36,19 @@ def build_query_string(kv_data, ignore_none=True):
     return query_string
 
 
+def strip_class_prefix(username, class_name):
+    """
+    去掉用户名里的 ks<班级号> 前缀，得到学生本人那一段。
+    用户名形如 ks251张三，class_name 为 251 时返回 张三。
+
+    用 removeprefix 而不是按长度切片：前缀对不上时原样返回，
+    不会从中间截出乱码。
+    """
+    if not class_name:
+        return username
+    return username.removeprefix(f"ks{class_name}")
+
+
 def datetime2str(value, format="iso-8601"):
     if format.lower() == "iso-8601":
         value = value.isoformat()