I have been doing a bit more research into this as I appeared to be a bit of a lone voice!
It is true that videos must be stored securely, but there is a bit more to be considered than storage.
Local Authorities should have a policy on Photography and Filming in relation to data protection. This is where schools are given advice on how to write their own policies.
Now that there is concern about safeguarding, a lot of schools are adopting the staff protocol that phones belonging to staff are not used in lessons, in any form, including taking images. This is quite normal and, OP, I would check what your school's policy is on this. It is the same for pupil's phones by the way. Only school owned devices should be used.
Also, you have probably given consent for photography and filming, but this consent rarely covers every eventuality. If the school has said it is for publicity, notice boards in school, newsletter and media use, then this is only what it is for. It is not for proving a teacher has adequate class control. This is a change to consent you gave and should have been the subject of a separate request. Can you find what you signed, OP, and check they have followed their agreement with parents? If not, the Head should be informed without delay. Unexpected use of video images is not acceptable. Parents should be told what the images are used for and the consequences of their use. It is perfectly possible for images to be used for educational reasons, but you must be told of this in the consent form and agree to it. A school will normally not use a child's image for any purpose if it prejudices the interests of the child.
In addition, children who did not want to be filmed should have been asked if they wished to move out of shot. If a child did not like being filmed, then they should have been given the option to move away. Was this option given, OP?
If children were filmed and it caused them distress, this is not permitted either. Only you can judge that.
I would ask the school what their policy states regarding staff protocol on personal devices and filming in classrooms. I would also check what you have agreed to and, more importantly, what you have not agreed to.
It is not teacher bashing to be clear and open with parents and children. If this was a man filming on his own phone, would everyone be so happy?