diff --git a/client/src/pages/joinClass.vue b/client/src/pages/joinClass.vue
index 2f83c2ab..7318b0ad 100644
--- a/client/src/pages/joinClass.vue
+++ b/client/src/pages/joinClass.vue
@@ -2,17 +2,21 @@
   <div>
     <h1>Zugangscode eingeben</h1>
     <div>
-      <div v-if="error">
-        {{error}}
-      </div>
       <div class="skillboxform-input">
         <label for="join-code" class="skillboxform-input__label">Zugangscode</label>
         <input
           id="join-code"
           class="skillbox-input skillboxform-input__input"
+          :class="{'skillboxform-input__input--error': error}"
           data-cy="input-class-code"
           :value="code"
           @input="updateCode">
+        <small
+          v-if="error"
+          class="skillboxform-input__error"
+          data-cy="email-local-errors"
+        >{{ error }}
+        </small>
       </div>
 
       <div>
@@ -65,8 +69,11 @@
           })
           .catch(e => {
             console.debug(e);
-            console.error(e.message);
-            this.error = 'Code ist nicht gültig';
+            if (e.message.indexOf('[CAJ]') > -1) {
+              this.error = 'Klasse bereits beigetreten';
+            } else {
+              this.error = 'Code ist nicht gültig';
+            }
           })
       },
       cancel() {
diff --git a/server/users/mutations.py b/server/users/mutations.py
index 8ce94560..96490bc9 100644
--- a/server/users/mutations.py
+++ b/server/users/mutations.py
@@ -120,13 +120,17 @@ class JoinClass(relay.ClientIDMutation):
         user = info.context.user
         code = kwargs.get('code')
         try:
-            school_class = SchoolClass.objects.get(Q(code__iexact=code) & ~Q(users=user))
+            school_class = SchoolClass.objects.get(Q(code__iexact=code))
 
-            school_class.users.add(user)
+
+            if user not in list(school_class.users.all()):
+                school_class.users.add(user)
+            else:
+                raise CodeNotFoundException('[CAJ] Schüler ist bereits in Klasse') # CAJ = Class Already Joined
 
             return cls(success=True, school_class=school_class)
         except SchoolClass.DoesNotExist:
-            raise CodeNotFoundException('Code ist nicht gültig')
+            raise CodeNotFoundException('[CNV] Code ist nicht gültig') # CAV = Code Not Valid
 
 
 class ProfileMutations: