diff --git a/backend/src/Administration/Application/Command/ChangeStudentClass/ChangeStudentClassHandler.php b/backend/src/Administration/Application/Command/ChangeStudentClass/ChangeStudentClassHandler.php
index f881069..af5e21d 100644
--- a/backend/src/Administration/Application/Command/ChangeStudentClass/ChangeStudentClassHandler.php
+++ b/backend/src/Administration/Application/Command/ChangeStudentClass/ChangeStudentClassHandler.php
@@ -4,7 +4,6 @@ declare(strict_types=1);
 
 namespace App\Administration\Application\Command\ChangeStudentClass;
 
-use App\Administration\Domain\Exception\AffectationEleveNonTrouveeException;
 use App\Administration\Domain\Exception\ClasseNotFoundException;
 use App\Administration\Domain\Model\ClassAssignment\ClassAssignment;
 use App\Administration\Domain\Model\SchoolClass\AcademicYearId;
@@ -44,15 +43,23 @@ final readonly class ChangeStudentClassHandler
             throw ClasseNotFoundException::withId($newClassId);
         }
 
-        // Trouver l'affectation existante
+        $now = $this->clock->now();
+
+        // Trouver l'affectation existante ou en créer une nouvelle
         $assignment = $this->classAssignmentRepository->findByStudent($studentId, $academicYearId, $tenantId);
 
-        if ($assignment === null) {
-            throw AffectationEleveNonTrouveeException::pourEleve($studentId);
+        if ($assignment !== null) {
+            $assignment->changerClasse($newClassId, $now);
+        } else {
+            $assignment = ClassAssignment::affecter(
+                tenantId: $tenantId,
+                studentId: $studentId,
+                classId: $newClassId,
+                academicYearId: $academicYearId,
+                assignedAt: $now,
+            );
         }
 
-        $assignment->changerClasse($newClassId, $this->clock->now());
-
         $this->classAssignmentRepository->save($assignment);
 
         return $assignment;
diff --git a/backend/src/Administration/Domain/Exception/AffectationEleveNonTrouveeException.php b/backend/src/Administration/Domain/Exception/AffectationEleveNonTrouveeException.php
deleted file mode 100644
index 4ff90c5..0000000
--- a/backend/src/Administration/Domain/Exception/AffectationEleveNonTrouveeException.php
+++ /dev/null
@@ -1,21 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-namespace App\Administration\Domain\Exception;
-
-use App\Administration\Domain\Model\User\UserId;
-use DomainException;
-
-use function sprintf;
-
-final class AffectationEleveNonTrouveeException extends DomainException
-{
-    public static function pourEleve(UserId $studentId): self
-    {
-        return new self(sprintf(
-            'Aucune affectation trouvée pour l\'élève "%s" cette année scolaire.',
-            $studentId,
-        ));
-    }
-}
diff --git a/backend/src/Administration/Infrastructure/Api/Processor/ChangeStudentClassProcessor.php b/backend/src/Administration/Infrastructure/Api/Processor/ChangeStudentClassProcessor.php
index 3a84555..bed05a3 100644
--- a/backend/src/Administration/Infrastructure/Api/Processor/ChangeStudentClassProcessor.php
+++ b/backend/src/Administration/Infrastructure/Api/Processor/ChangeStudentClassProcessor.php
@@ -8,7 +8,6 @@ use ApiPlatform\Metadata\Operation;
 use ApiPlatform\State\ProcessorInterface;
 use App\Administration\Application\Command\ChangeStudentClass\ChangeStudentClassCommand;
 use App\Administration\Application\Command\ChangeStudentClass\ChangeStudentClassHandler;
-use App\Administration\Domain\Exception\AffectationEleveNonTrouveeException;
 use App\Administration\Domain\Exception\ClasseNotFoundException;
 use App\Administration\Domain\Repository\ClassRepository;
 use App\Administration\Infrastructure\Api\Resource\StudentResource;
@@ -80,8 +79,6 @@ final readonly class ChangeStudentClassProcessor implements ProcessorInterface
             $data->classLevel = $newClass->level?->value;
 
             return $data;
-        } catch (AffectationEleveNonTrouveeException) {
-            throw new NotFoundHttpException('Élève non trouvé.');
         } catch (ClasseNotFoundException $e) {
             throw new NotFoundHttpException($e->getMessage());
         }
diff --git a/backend/tests/Unit/Administration/Application/Command/ChangeStudentClass/ChangeStudentClassHandlerTest.php b/backend/tests/Unit/Administration/Application/Command/ChangeStudentClass/ChangeStudentClassHandlerTest.php
index 3a7216f..509a776 100644
--- a/backend/tests/Unit/Administration/Application/Command/ChangeStudentClass/ChangeStudentClassHandlerTest.php
+++ b/backend/tests/Unit/Administration/Application/Command/ChangeStudentClass/ChangeStudentClassHandlerTest.php
@@ -6,7 +6,6 @@ namespace App\Tests\Unit\Administration\Application\Command\ChangeStudentClass;
 
 use App\Administration\Application\Command\ChangeStudentClass\ChangeStudentClassCommand;
 use App\Administration\Application\Command\ChangeStudentClass\ChangeStudentClassHandler;
-use App\Administration\Domain\Exception\AffectationEleveNonTrouveeException;
 use App\Administration\Domain\Exception\ClasseNotFoundException;
 use App\Administration\Domain\Model\ClassAssignment\ClassAssignment;
 use App\Administration\Domain\Model\SchoolClass\AcademicYearId;
@@ -74,13 +73,17 @@ final class ChangeStudentClassHandlerTest extends TestCase
     }
 
     #[Test]
-    public function itThrowsWhenAssignmentNotFound(): void
+    public function itCreatesAssignmentWhenNoneExists(): void
     {
         $handler = $this->createHandler();
-        $command = $this->createCommand(studentId: '550e8400-e29b-41d4-a716-446655440070');
+        $newStudentId = '550e8400-e29b-41d4-a716-446655440070';
+        $command = $this->createCommand(studentId: $newStudentId);
 
-        $this->expectException(AffectationEleveNonTrouveeException::class);
-        $handler($command);
+        $assignment = $handler($command);
+
+        self::assertTrue($assignment->studentId->equals(UserId::fromString($newStudentId)));
+        self::assertTrue($assignment->classId->equals(ClassId::fromString(self::NEW_CLASS_ID)));
+        self::assertTrue($assignment->academicYearId->equals(AcademicYearId::fromString(self::ACADEMIC_YEAR_ID)));
     }
 
     #[Test]
diff --git a/frontend/src/routes/admin/students/+page.svelte b/frontend/src/routes/admin/students/+page.svelte
index d31c0a2..676bccd 100644
--- a/frontend/src/routes/admin/students/+page.svelte
+++ b/frontend/src/routes/admin/students/+page.svelte
@@ -62,6 +62,7 @@
 	let changeClassTarget = $state<Student | null>(null);
 	let newClassForChange = $state('');
 	let isChangingClass = $state(false);
+	let changeClassError = $state<string | null>(null);
 
 	// Classes grouped by level for optgroup
 	let classesByLevel = $derived.by(() => {
@@ -300,7 +301,7 @@
 		changeClassTarget = student;
 		newClassForChange = '';
 		showChangeClassModal = true;
-		error = null;
+		changeClassError = null;
 	}
 
 	function closeChangeClassModal() {
@@ -338,7 +339,7 @@
 			successMessage = `${changeClassTarget.firstName} ${changeClassTarget.lastName} a été transféré vers ${targetClass?.name ?? 'la nouvelle classe'}.`;
 			closeChangeClassModal();
 		} catch (e) {
-			error = e instanceof Error ? e.message : 'Erreur lors du changement de classe';
+			changeClassError = e instanceof Error ? e.message : 'Erreur lors du changement de classe';
 		} finally {
 			isChangingClass = false;
 		}
@@ -720,6 +721,13 @@
 					</select>
 				</div>
 
+				{#if changeClassError}
+					<div class="alert alert-error modal-error">
+						{changeClassError}
+						<button class="alert-close" onclick={() => (changeClassError = null)}>×</button>
+					</div>
+				{/if}
+
 				{#if newClassForChange}
 					{@const targetClass = classes.find((c) => c.id === newClassForChange)}
 					<div class="change-confirm-info">
@@ -1244,6 +1252,10 @@
 		justify-content: flex-end;
 	}
 
+	.modal-error {
+		margin-bottom: 0;
+	}
+
 	.change-confirm-info {
 		padding: 0.75rem 1rem;
 		background: #eff6ff;