Oh OP, I feel your pain. I'm not school-based now but some years ago I ended up being an almost full-time teaching deputy head in a year 6 class (that had a third of its pupils at risk of permanent exclusion), assessment coordinator, Literacy coord, music coord, conductor of school choir ... I was pulling 90 hour weeks at one point and, needless to say, it was completely unsustainable ...
To add to what others have suggested, perhaps go the 'logic' route but with the angle of safeguarding and potential implications should there be a sudden concern surrounding a LAC or CP ie a child makes a disclosure. I'm presuming that if something happened early on on the day you were teaching, you'd have to deal with this as a matter of urgency and this would impact on others' PPA. Of course, the head may argue that the teachers who you were covering would have to return to class and then have cover at another time.
Are there high numbers of pupils on the at risk register?
It's best to try to completely take your feelings out of the equation when speaking to the head and stick only to facts as in,
'I've had the weekend to think about the new arrangement we discussed on Friday and I have a few concerns...'
Avoid any 'I feel...' statements (easier said than done) and try to present facts from an objective viewpoint.
Appeal to the head's better nature (ahem, if there is one)
For you!