People in a public place do not normally have a right to privacy, so they can’t legally prevent you from filming: though it’s advisable not to film anyone who clearly doesn’t want to be filmed.
The exception is where people could reasonably expect privacy, for example if they are being treated after an accident.
You should also be very careful about filming children and vulnerable adults: it’s not technically illegal in a public place, but you may be challenged and your motives may be questioned.
If you include children and young people in your film (eg pupils) you should only use their first names, and not include any other information which might help people identify or locate them. You should get parental permission before distributing or disseminating the film. It’s best to get this before you start filming. Many schools ask parents to sign a blanket permission form which may be adequate, but you should check that this covers all the children in the film (and be aware of any students who should not be filmed for child protection reasons).
In a private place (which includes most shopping malls) there is no right to film without permission and you are likely to be stopped by security guards. It is also illegal to film on railway stations, in airports and in some other locations.