Can I ask a stupid question - why wouldn't you log flying hours because the school decided not to charge you for them?
In my case since I was at an intensive course, and I knew that at the end of the course the CFI would pull the financial records from the accounting department and compare them to what was in my logbook before signing the entries off as "logbook hours correct".
My estimate was that the time it would take me to explain this would be more than the actual time in my logbook, so the most efficient way was not to mention them.
(Never knew that five years down the line PPRuNe would come down to haunt me anyway...
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