diff --git a/src/admin/setting/config.vue b/src/admin/setting/config.vue
index 7901223..23fdc42 100644
--- a/src/admin/setting/config.vue
+++ b/src/admin/setting/config.vue
@@ -1,5 +1,10 @@
 <script setup lang="ts">
 import { NButton, NTag } from "naive-ui"
+import {
+  CLASS_NAME_MAX_DIGITS,
+  CLASS_NAME_MIN_DIGITS,
+  CLASS_NAME_RE,
+} from "utils/constants"
 import { parseTime } from "utils/functions"
 import type { Server } from "utils/types"
 import { useConfigStore } from "shared/store/config"
@@ -140,7 +145,21 @@ async function getWebsiteConfig() {
 }
 
 async function saveWebsiteConfig() {
-  await editWebsite(websiteConfig)
+  // 班级号要和用户名里 ks 后面那段对得上，位数不对登录页会查不到该班学生。
+  // 后端 CreateEditWebsiteConfigSerializer 也会拦，这里先报更明确的错
+  const invalid = websiteConfig.class_list.filter((c) => !CLASS_NAME_RE.test(c))
+  if (invalid.length) {
+    message.error(
+      `班级号 ${invalid.join("、")} 必须是 ${CLASS_NAME_MIN_DIGITS}~${CLASS_NAME_MAX_DIGITS} 位数字`,
+    )
+    return
+  }
+  try {
+    await editWebsite(websiteConfig)
+  } catch (err: any) {
+    message.error("保存失败：" + err.data)
+    return
+  }
   message.success("网站配置保存成功")
   getWebsiteConfig()
   configStore.getConfig()
@@ -214,7 +233,13 @@ onMounted(() => {
     </n-form>
     <n-form label-placement="left">
       <n-form-item label="班级列表">
-        <n-dynamic-tags v-model:value="websiteConfig.class_list" />
+        <n-flex vertical size="small">
+          <n-dynamic-tags v-model:value="websiteConfig.class_list" />
+          <n-text depth="3" style="font-size: 12px">
+            填 {{ CLASS_NAME_MIN_DIGITS }}~{{ CLASS_NAME_MAX_DIGITS }}
+            位数字，如 251、2510，要和用户名里 ks 后面那段一致
+          </n-text>
+        </n-flex>
       </n-form-item>
     </n-form>
     <n-flex align="center">
diff --git a/src/admin/user/generate.vue b/src/admin/user/generate.vue
index 88b7740..d613f3b 100644
--- a/src/admin/user/generate.vue
+++ b/src/admin/user/generate.vue
@@ -1,4 +1,10 @@
 <script setup lang="ts">
+import {
+  CLASS_NAME_MAX_DIGITS,
+  CLASS_NAME_MAX_VALUE,
+  CLASS_NAME_MIN_DIGITS,
+  CLASS_NAME_MIN_VALUE,
+} from "utils/constants"
 import { importUsers } from "../api"
 
 const message = useMessage()
@@ -12,6 +18,17 @@ function generateUsers() {
     message.info("请填写相关内容")
     return false
   }
+  // 后端 get_class_name 只认合法位数的班级号，位数不对会整批拒绝导入，
+  // 这里先拦一道，省得填完一屏用户名才被打回来
+  if (
+    prefix.value &&
+    (prefix.value < CLASS_NAME_MIN_VALUE || prefix.value > CLASS_NAME_MAX_VALUE)
+  ) {
+    message.error(
+      `班级号 ${prefix.value} 必须是 ${CLASS_NAME_MIN_DIGITS}~${CLASS_NAME_MAX_DIGITS} 位数字`,
+    )
+    return false
+  }
   let className = !!prefix.value ? `ks${prefix.value}` : ""
   rawInput.value = rawInput.value.trim()
   const inputs = rawInput.value.split("\n")
@@ -64,9 +81,9 @@ async function submit() {
           style="width: 170px"
           v-model:value="prefix"
           clearable
-          :max="9999"
+          :max="CLASS_NAME_MAX_VALUE"
           :min="0"
-          placeholder="班级号"
+          :placeholder="`班级号（${CLASS_NAME_MIN_DIGITS}~${CLASS_NAME_MAX_DIGITS} 位）`"
         />
       </n-flex>
       <n-input
diff --git a/src/admin/user/list.vue b/src/admin/user/list.vue
index 866f6e8..bc49bae 100644
--- a/src/admin/user/list.vue
+++ b/src/admin/user/list.vue
@@ -196,20 +196,29 @@ async function handleEditUser() {
     message.error("密码长度不得小于 6")
     return
   }
-  if (create.value) {
-    const newUser = [
-      [
-        userEditing.value.username,
-        password.value,
-        userEditing.value.email,
-        userEditing.value.real_name,
-      ],
-    ]
-    await importUsers(newUser)
-    listUsers()
-  } else {
-    const user = Object.assign(userEditing.value, { password: password.value })
-    await editUser(user)
+  // http 拦截器只对 login-required / permission-denied 自动弹提示，
+  // 其余业务错误（比如班级号位数不对）不接住就什么都不显示
+  try {
+    if (create.value) {
+      const newUser = [
+        [
+          userEditing.value.username,
+          password.value,
+          userEditing.value.email,
+          userEditing.value.real_name,
+        ],
+      ]
+      await importUsers(newUser)
+      listUsers()
+    } else {
+      const user = Object.assign(userEditing.value, {
+        password: password.value,
+      })
+      await editUser(user)
+    }
+  } catch (err: any) {
+    message.error("保存失败：" + err.data)
+    return
   }
   userEditing.value = null
   password.value = ""
diff --git a/src/oj/submission/components/ButtonWithSearch.vue b/src/oj/submission/components/ButtonWithSearch.vue
index 5579dbb..6e70507 100644
--- a/src/oj/submission/components/ButtonWithSearch.vue
+++ b/src/oj/submission/components/ButtonWithSearch.vue
@@ -1,5 +1,6 @@
 <script lang="ts" setup>
 import { Icon } from "@iconify/vue"
+import { USERNAME_CLASS_RE } from "utils/constants"
 
 interface Props {
   type: "题目" | "用户"
@@ -10,12 +11,14 @@ const props = defineProps<Props>()
 
 const emits = defineEmits(["click", "search", "filterClass"])
 
+// 用同一个正则判断，避免 kstest 这类 ks 开头但没有班级号的用户名
+// 也显示出按钮，点了却什么都不发生
 const showFilterClass = computed(() => {
-  return props.type === "用户" && props.username?.startsWith("ks")
+  return props.type === "用户" && USERNAME_CLASS_RE.test(props.username ?? "")
 })
 
 function filterClass() {
-  const match = props.username!.match(/^ks\d{3,4}/)
+  const match = props.username!.match(USERNAME_CLASS_RE)
   const classname = match ? match[0] : ""
   if (!classname) return
   emits("filterClass", classname)
diff --git a/src/utils/constants.ts b/src/utils/constants.ts
index 7cd2551..1c7ad81 100644
--- a/src/utils/constants.ts
+++ b/src/utils/constants.ts
@@ -325,3 +325,24 @@ export const DURATION_OPTIONS = [
   { label: "半年内", value: "months:6" },
   { label: "一年内", value: "years:1" },
 ] as const
+
+// 班级号的位数范围。学生用户名形如 ks<班级号><姓名>，班级号还要跟
+// 网站配置里的班级列表对得上。
+// 后端 OnlineJudge/utils/shortcuts.py 的 CLASS_NAME_MIN/MAX_DIGITS
+// 是同一条规则的另一份，改这里必须同时改那边。
+export const CLASS_NAME_MIN_DIGITS = 3
+export const CLASS_NAME_MAX_DIGITS = 4
+
+/** 合法班级号：3~4 位纯数字 */
+export const CLASS_NAME_RE = new RegExp(
+  `^\\d{${CLASS_NAME_MIN_DIGITS},${CLASS_NAME_MAX_DIGITS}}$`,
+)
+
+/** 用户名开头的 ks<班级号>，用于从用户名里认出班级 */
+export const USERNAME_CLASS_RE = new RegExp(
+  `^ks\\d{${CLASS_NAME_MIN_DIGITS},${CLASS_NAME_MAX_DIGITS}}`,
+)
+
+/** 班级号作为数字时的上下界，给 n-input-number 用 */
+export const CLASS_NAME_MIN_VALUE = 10 ** (CLASS_NAME_MIN_DIGITS - 1)
+export const CLASS_NAME_MAX_VALUE = 10 ** CLASS_NAME_MAX_DIGITS - 1