When you have a vulnerable person in the classroom, and masks have been directed because of local lockdown, you cannot put that person at risk.
The consequence of illness (which for the very vulnerable can be severe) outweigh temporary reasonable adjustments. The school cannot knowingly put it's more vulnerable community knowingly at risk because of the proximity of someone who cannot use the mitigations that are directed for a while.
It is a balance between the needs of two groups. You cannot put the vulnerable at risk of contagion (masks protect others, not the individual wearing/not wearing) so an alternative mitigation needs to be found.
Shielding in full form has essentially been abolished, so children who are OK for school, but have underlying medical conditions, could well be present. As well as those who are vulnerable (but short of previous shielding) plus vulnerable adults (flu jab, including pregnancy). Those people must not be put at risk either.
It's a difficult balance, as all relevant medical conditions are likely to be covered by DDA.
But risk of serious illness to others is never a reasonable adjustment, especially not when restrictions are a temporary measure during a time of heightened risk of transmission (condition described in first sentence of OP)