diff --git a/apps/api/src/routes/admin/account.ts b/apps/api/src/routes/admin/account.ts
index 49adbe9..d3f5c81 100644
--- a/apps/api/src/routes/admin/account.ts
+++ b/apps/api/src/routes/admin/account.ts
@@ -34,7 +34,7 @@ import { revokeUserSessions } from "../../auth/session"
 import { requireSuperAdmin, type AppEnv } from "../../auth/middleware"
 import { db, schema } from "../../db"
 import { failure, parseBody, success } from "../../http"
-import { queryInteger, sampleUser } from "../helpers"
+import { classPrefixCondition, queryInteger, sampleUser } from "../helpers"
 
 export const adminAccountRoutes = new Hono<AppEnv>()
 
@@ -130,7 +130,11 @@ adminAccountRoutes.get("/rankings/users", requireSuperAdmin, async (c) => {
   const where = and(
     inArray(schema.user.adminType, [...STUDENT_ROLES]),
     eq(schema.user.isDisabled, false),
-    keyword ? ilike(schema.user.username, `%${keyword}%`) : undefined,
+    // 输入班级前缀（ks251）按班级匹配，否则照旧按用户名包含
+    keyword
+      ? (classPrefixCondition(keyword) ??
+          ilike(schema.user.username, `%${keyword}%`))
+      : undefined,
   )
 
   const [totalRows, rows] = await Promise.all([
diff --git a/apps/api/src/routes/admin/conf.ts b/apps/api/src/routes/admin/conf.ts
index 355fa05..8e59ce6 100644
--- a/apps/api/src/routes/admin/conf.ts
+++ b/apps/api/src/routes/admin/conf.ts
@@ -26,7 +26,7 @@ import { failure, parseBody, success } from "../../http"
 import { sniffImageExtension } from "../../services/image"
 import { getWebsiteOptions } from "../../services/options"
 import { dayStart } from "../../time"
-import { queryInteger } from "../helpers"
+import { classPrefixCondition, queryInteger } from "../helpers"
 
 export const adminConfRoutes = new Hono<AppEnv>()
 
@@ -253,14 +253,17 @@ adminConfRoutes.get("/dashboard", requireSuperAdmin, async (c) => {
 
 adminConfRoutes.get("/random-usernames", requireSuperAdmin, async (c) => {
   // 传的是**班级前缀**（形如 ks251），不是班级号 —— 前端输入框写的就是「班级前缀」，
-  // 拿到结果后按这个前缀 split 取姓名。这里按前缀匹配，与旧 istartswith 一致，
-  // 不额外按 className 过滤：那会改变旧行为，而这个功能就是随机点名，宁可宽松
+  // 拿到结果后按这个前缀 split 取姓名。班级前缀走 classPrefixCondition（ks251 不带上
+  // ks2511）；别的形状的输入照旧 istartswith，与旧接口一致
   const classroom = c.req.query("classroom")?.trim()
   if (!classroom) return failure(c, 400, "invalid-request", "需要班级号")
   const rows = await db
     .select({ username: schema.user.username })
     .from(schema.user)
-    .where(ilike(schema.user.username, `${classroom}%`))
+    .where(
+      classPrefixCondition(classroom) ??
+        ilike(schema.user.username, `${classroom}%`),
+    )
     .orderBy(sql`random()`)
     .limit(10)
   return success(
diff --git a/apps/api/src/routes/helpers.ts b/apps/api/src/routes/helpers.ts
index 0ab5a20..a6c65f5 100644
--- a/apps/api/src/routes/helpers.ts
+++ b/apps/api/src/routes/helpers.ts
@@ -1,6 +1,6 @@
 import { ADMIN_ROLES, TEACHER_ROLES, type SampleUser } from "@oj2/contract"
 
-import { and, count, eq, ilike, notInArray } from "drizzle-orm"
+import { and, count, eq, ilike, notInArray, sql } from "drizzle-orm"
 
 import type { AuthUser } from "../auth/session"
 import { db, schema } from "../db"
@@ -44,6 +44,22 @@ export function stripClassPrefix(
   return username.startsWith(prefix) ? username.slice(prefix.length) : username
 }
 
+/**
+ * 用户名按「班级前缀」匹配：`ks251` 只匹配 251 班，**不能**把 `ks2511张三` 也捞进来。
+ *
+ * 班级号 3~4 位并存（见 admin/account.ts 的 classNameOf），裸 `ilike 'ks251%'` 会把
+ * 2510~2519 班全算进 251 班。所以输入是 `ks` + 3 位以上数字时，要求紧跟着的不是数字；
+ * 1~2 位（`ks25`）是年级前缀，照旧宽松匹配。输入不是 `ks+数字` 形状时返回 undefined，
+ * 由调用方决定退回什么匹配方式。
+ */
+export function classPrefixCondition(input: string) {
+  const matched = /^ks(\d+)$/i.exec(input)
+  if (!matched) return undefined
+  const prefix = ilike(schema.user.username, `${input}%`)
+  if (matched[1]!.length < 3) return prefix
+  return and(prefix, sql`${schema.user.username} !~* ${`^${input}[0-9]`}`)
+}
+
 /**
  * 拿 query 里的筛选值去比对 `$type` 收窄过的列（`submission.result`、`problem.difficulty` 这些）。
  *
diff --git a/apps/api/src/routes/problemset.ts b/apps/api/src/routes/problemset.ts
index c1c617f..b1644dc 100644
--- a/apps/api/src/routes/problemset.ts
+++ b/apps/api/src/routes/problemset.ts
@@ -33,7 +33,13 @@ import {
 import { db, schema } from "../db"
 import { failure, parseBody, success } from "../http"
 import { computeProgress } from "../services/problemset"
-import { asFilterValue, asRecord, queryInteger, sampleUser } from "./helpers"
+import {
+  asFilterValue,
+  asRecord,
+  classPrefixCondition,
+  queryInteger,
+  sampleUser,
+} from "./helpers"
 
 export const problemsetRoutes = new Hono<AppEnv>()
 
@@ -499,7 +505,15 @@ problemsetRoutes.get(
     const className = c.req.query("className")?.trim()
     const completion = c.req.query("completionStatus")?.trim()
     const filters = [eq(schema.problemsetProgress.problemsetId, id)]
-    if (className) filters.push(ilike(schema.user.username, `%${className}%`))
+    // 只填数字（251）当班级号补上 ks；班级按 classPrefixCondition 匹配，
+    // 否则 251 会把 2511 班也算进来。别的输入照旧按用户名包含
+    if (className) {
+      const classPrefix = /^\d+$/.test(className) ? `ks${className}` : className
+      filters.push(
+        classPrefixCondition(classPrefix) ??
+          ilike(schema.user.username, `%${className}%`),
+      )
+    }
     if (completion === "completed")
       filters.push(eq(schema.problemsetProgress.isCompleted, true))
     else if (completion === "in_progress")