diff --git a/api/views.py b/api/views.py
index b4c97b45..ec936208 100644
--- a/api/views.py
+++ b/api/views.py
@@ -14,7 +14,6 @@
     Semester,
     Subject,
     assignedtask_results,
-    current_semester_conds,
     current_semester,
     submit_assignment_path,
 )
@@ -202,13 +201,17 @@ def sort_fn(c):
 
 
 @user_passes_test(is_teacher)
-def subject_list(request, subject_abbr):
+def subject_list(request, subject_abbr: str):
+    """
+    Returns the list of active classes for a given subject.
+    Used when creating a new task.
+    """
     get_object_or_404(
         Subject, abbr=subject_abbr
     )  # The result is not needed, the call is to provide 404 error
 
     classes = []
-    for clazz in Class.objects.filter(subject__abbr=subject_abbr, **current_semester_conds()):
+    for clazz in Class.objects.current_semester().filter(subject__abbr=subject_abbr):
         classes.append(
             {
                 "id": clazz.pk,
@@ -515,9 +518,8 @@ def is_allowed(path):
                     "type": "file",
                 }
 
-    classes = Class.objects.filter(
+    classes = Class.objects.current_semester().filter(
         subject__abbr=task.subject.abbr,
-        **current_semester_conds(),
     )
     assigned_count = 0
     for clazz in classes:
diff --git a/common/migrations/0020_semester_active.py b/common/migrations/0020_semester_active.py
new file mode 100644
index 00000000..b4b23aa5
--- /dev/null
+++ b/common/migrations/0020_semester_active.py
@@ -0,0 +1,18 @@
+# Generated by Django 4.2.16 on 2024-09-08 07:22
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('common', '0019_submit_ip_address'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='semester',
+            name='active',
+            field=models.BooleanField(default=False),
+        ),
+    ]
diff --git a/common/models.py b/common/models.py
index 2e5133c3..be26060d 100644
--- a/common/models.py
+++ b/common/models.py
@@ -2,8 +2,9 @@
 import re
 import logging
 
-from typing import List
+from typing import List, Optional
 
+from django.db.models import QuerySet
 from django.utils import timezone
 
 from django.db import models
@@ -16,25 +17,34 @@
 from jinja2 import Environment, FileSystemLoader
 
 
-def current_semester_conds(prefix=""):
-    return {
-        f"{prefix}semester__begin__lte": timezone.now(),
-        f"{prefix}semester__end__gte": timezone.now(),
-    }
-
-
-def current_semester() -> "Semester":
-    semester = Semester.objects.filter(begin__lte=timezone.now(), end__gte=timezone.now()).first()
+def current_semester() -> Optional["Semester"]:
+    """
+    Returns the current active semester, if there is any.
+    """
+    semester = Semester.objects.filter(active=True).order_by("-begin").first()
 
     if semester:
         return semester
 
-    return Semester.objects.filter(begin__lte=timezone.now()).order_by("begin").last()
+    # If no semester is marked as active, return the latest semester that has already begun.
+    return Semester.objects.filter(begin__lte=timezone.now()).order_by("-begin").first()
 
 
 class ClassManager(models.Manager):
-    def current_semester(self):
-        return self.filter(**current_semester_conds())
+    def current_semester(self) -> QuerySet:
+        """
+        Return classes for the currently active semester.
+        Note that the semantics for this call are a bit less strict than for `current_semester`.
+        Notably, if multiple semesters are active, it will return classes for all of them.
+
+        We could create some query like
+        WHERE r.active=1 AND NOT
+        EXISTS (SELECT * FROM semester AS s WHERE s.active=1 AND r.id != s.id AND s.begin > r.begin)
+
+        But it seems like overkill, since we will only ever have exactly one active semester
+        (we just need to make sure in admin that this property holds).
+        """
+        return self.filter(semester__active=True)
 
 
 class Semester(models.Model):
@@ -42,6 +52,9 @@ class Semester(models.Model):
     end = models.DateField()
     year = models.IntegerField()
     winter = models.BooleanField()
+    # Is the semester currently marked as active?
+    # Ideally, only one semester should be marked as such.
+    active = models.BooleanField(default=False)
 
     def __str__(self):
         return f"{self.year}{'W' if self.winter else 'S'}"