Best to ask HR about how much you'd lose. If you were full time you'd lose 1/365th not the 1/195th that you would actually be absent for. So how they work out part time is probably complicated. Out of curiosity, what would be the justification for putting a grievance in? I ask because at the moment I bend the rules all the time. For example, if it were my school, I would give the time off with leave if you could make up the hours at another time. But that would also depend upon the knock on effect of others e.g changes to PPA timings/timetables etc. My main concern with bending the rules is that if there comes an instance when I can't, then I might be leaving myself open to a grievance.