As can be seen, whatever was originally written underneath 504 hrs was rubbed out and the words 'a month' added. It could be that this correction failed to take into account the original basis of the 504 hours figure. So for instance during the Easter holidays she only worked for 3 weeks, whereas in the summer holidays she worked for 6 weeks, making 9 weeks per year in all. That is over 2 months, but say the total number of hours she worked was 1010 a year and someone just divided that by two and got 504, whereas in fact it was 1010 hours over 2 months and one week (or 63 days) which then works out at 15 hours per day for a 7 day week. Yes, still quite long hours for someone who was presumably a school girl (or school teacher?) but only for relatively short stints. It may be some of those hours were a night shift when she might have got some sleep while still being on call.
Incidentally even if it really was 504 hours per month, that is 16.25 hours per day for a 31 day month and 16.8 hours for a 30 day month, not 18 hours as you stated, assuming she worked 7 days a week.