We have a parents in classrooms procedure for drop off in infants. For our school it is what works well for us, our children and our parents. It is not something we would want to change at present. It is a nice point of contact for parents, pupils and staff in the morning.
However, every class has two members of staff present in a morning - teacher and TA. Doors are watched, children are acknowledged and have a morning task to engage themselves in.
In reception we have a "welcome door" rota. A member of staff is present by the door to reception to nominally greet parents and children as they arrive, but security wise to check that children do not leave school unaccompanied.
Once the bell goes all parents must have gone and the doors are on automatically lock. In reception the button to press to open the door is completely out of a child's reach. The welcome door duty person remains there for a further 5-10 minutes to check all parents have left, and then notifies the reception classes once she/he leaves.
In KS1 the doors are locked to prevent outside access without being buzzed in or by using a fob. Children can/ however, get out if they wanted to. They are talked about not leaving of course. However, the school gates are on an automatic lock also and also on CCTV with the screens in the main office.
Nothing is fallible without a security guard in the gates all day - but that just isn't going to happen tbh. Children are talked to about security and staff are continually trained and reminded. OFSTED found all our procedures secure and safe, including morning drop off with parents in school. Infact they liked the relaxed welcoming approach. So it doesn't have to be stopped and it doesn't have to be a negative. It can work - but required staff in numbers, training and continued training and reminders - and that goes for staff and pupils.