diff --git a/app/controllers/Clas.scala b/app/controllers/Clas.scala
index 3c3434432d..e51609d7ab 100644
--- a/app/controllers/Clas.scala
+++ b/app/controllers/Clas.scala
@@ -134,7 +134,7 @@ final class Clas(
               html.clas.student.form(
                 clas,
                 students,
-                env.clas.forms.student.invite,
+                env.clas.forms.student.invite(clas),
                 createForm,
                 created
               )
@@ -156,7 +156,7 @@ final class Clas(
                   html.clas.student.form(
                     clas,
                     students,
-                    env.clas.forms.student.invite,
+                    env.clas.forms.student.invite(clas),
                     err
                   )
                 ).fuccess,
@@ -174,7 +174,8 @@ final class Clas(
 
   def studentInvite(id: String) = SecureBody(_.Teacher) { implicit ctx => me =>
     WithClassAndStudents(me, id) { t => (clas, students) =>
-      env.clas.forms.student.invite
+      env.clas.forms.student
+        .invite(clas)
         .bindFromRequest()(ctx.body)
         .fold(
           err =>
diff --git a/app/views/clas/clas.scala b/app/views/clas/clas.scala
index 050a99f5e1..389716c88b 100644
--- a/app/views/clas/clas.scala
+++ b/app/views/clas/clas.scala
@@ -5,7 +5,7 @@ import play.api.data.Form
 import lila.api.Context
 import lila.app.templating.Environment._
 import lila.app.ui.ScalatagsTemplate._
-import lila.clas.{ Clas, Student }
+import lila.clas.Clas
 import lila.clas.ClasForm.ClasData
 import controllers.routes
 
diff --git a/modules/clas/src/main/Clas.scala b/modules/clas/src/main/Clas.scala
index 1d1c474ded..b78a389381 100644
--- a/modules/clas/src/main/Clas.scala
+++ b/modules/clas/src/main/Clas.scala
@@ -8,7 +8,6 @@ case class Clas(
     name: String,
     desc: String,
     teachers: NonEmptyList[Teacher.Id], // first is owner
-    nbStudents: Int,
     created: Clas.Recorded,
     viewedAt: DateTime,
     archived: Option[Clas.Recorded]
@@ -29,7 +28,6 @@ object Clas {
     name = name,
     desc = desc,
     teachers = NonEmptyList(teacher.id),
-    nbStudents = 0,
     created = Recorded(teacher.id, DateTime.now),
     viewedAt = DateTime.now,
     archived = none
diff --git a/modules/clas/src/main/ClasForm.scala b/modules/clas/src/main/ClasForm.scala
index df4c535977..103bfbc6fe 100644
--- a/modules/clas/src/main/ClasForm.scala
+++ b/modules/clas/src/main/ClasForm.scala
@@ -28,7 +28,7 @@ final class ClasForm(
 
   object student {
 
-    def create: Form[NewStudent] =
+    val create: Form[NewStudent] =
       Form(
         mapping(
           "create-username" -> securityForms.signup.username,
@@ -44,12 +44,12 @@ final class ClasForm(
         )
     }
 
-    def invite =
+    def invite(c: Clas) =
       Form(
         mapping(
-          "username" -> lila.user.DataForm.historicalUsernameField.verifying("Unknown username", {
-            blockingFetchUser(_).isDefined
-          }),
+          "username" -> lila.user.DataForm.historicalUsernameField
+            .verifying("Unknown username", { blockingFetchUser(_).isDefined })
+            .verifying("This is a teacher", u => !c.teachers.toList.contains(u.toLowerCase)),
           "realName" -> nonEmptyText
         )(NewStudent.apply)(NewStudent.unapply)
       )