I'm not sure I'd agree with it.

If the idea is to restrict people moving into the area to get a place and the moving out once obtained, ok in principle though would be people just stay for 2 years?

in practice however what would happen? The first child remaining at school a and the second child going to school b? Getting them to school would be a nightmare for the parents.

Or you could try and move child a to school b, causing unneeded disruption.

I expect this won't work and perhaps need further thought as people do have genuine reasons to have to move.

The answer to this really is better town \ council planning, reducing the extreme shortage of places and bringing up the standard of schools generally (admittedly very good already in SE23).