How to exclude Django Modelform fields

models.py

class Child(models.Model):
name = models.CharField(max_length=200)
last_name = models.CharField(max_length=200)
grade = models.CharField(max_length=200)

admin.py

@admin.register(Child)
class ChildAdmin(admin.ModelAdmin):
exclude = ('grade',)

yields the same result as

@admin.register(Child)
class ChildAdmin(admin.ModelAdmin):
fields = ('name', 'last_name')