The DfE don't allow a student's results to be discounted from progress measures if they attend the school prior to Spring Census Day. They only allow them to be discounted if they
a) Join a school after Spring Census. The results will be included in the previous school's progress measures, so there is no benefit to doing this - and not entering them also shows up as negative progress.
b) Join a school after having been permanently excluded twice before joining them. One of the previous schools will have the results included in their measures.
c) Join the school prior to Spring Census Day AND are new to the country AND they are evidenced as coming from a country where it has not already been blanket banned from having results discounted because English is a common language - which means, due to the influence of the British Empire, the vast majority of countries that children tend to come to the UK from are ones where they cannot be excluded.
d) Are in youth custody, fully evidenced.
e) Are fully evidenced as being life threateningly or terminally ill and even then, not in all cases.
The school has to provide this information over the course of a week in July in the form of no more than six pdf pages in a fairly lengthy and convoluted process for each child, there is no right of appeal and it's largely dependent upon whether the person at the DfE (who have just taken over the operation from a private contractor) are able to handle the requests correctly. The results so far have not been promising.
They also cannot avoid the negative progress scores by excluding/putting the child into alternative provison/a PRU because they do not have progress measures - the results or lack of are still attached to the school.
The only time when it's vaguely non-detrimental to a school to encourage/cheat/break the Law is if the child did not sit KS2 SATs. However, as there are no formal progress measures for this year or next due to Covid, there's nothing to be gained from certain academy chains behaving like this at all.
Oh, and a school suddenly having a spike in permanent exclusions, managed moves and EHE in Y11 will attract the attention of both the LA and Ofsted, as illegal offrolling is an instant Inadequate. Which, combined with everything else makes actually PEXing somebody in Y11 extremely challenging, even if whatever they did was absolutely PEX-worthy. The only one I know of that stuck involved an extremely large knife and a somewhat traumatised member of staff - and yes, they did have SEND, yes, their attainment wasn't going to be great, but that was not something the school could say 'oh well, no biggie, see you Monday'.
I'm posting this information not as a criticism of you, but it somewhat counters the statements that some of the parents make.