Which shoe chain is this? I'm going to boycott them...
Had the same problem with McDonalds when I was younger: 30-hour week paid 20 (it was supposed to be a p/t job, to fit around my studies!)
I also knew someone who worked for Starbucks for 1 year: she had to do at least 2 hours unpaid overtime every day (and she was mostly cleaning and doing the washing up, even though her job title was 'barista'). She worked at an airport too, which meant hectic hours: she would finish work at 10pm, arrive home at 11pm, have tea and take a shower, then off to bed at midnight... only to get up at 2:30am in order to take the 3am shuttle to the airport and start a new shift (minimum 10 hours) at 4am...
If I were your granddaughter, I'd drop that job and concentrate on my A levels.