Its unlikely that you would fail in the spelling and grammar if you are confident that you are good, but there could be other areas they are assessing you on too.
It is possible that this is a group interview, followed by an individual interview. I have been to interviews such as this, not in Debanhams though. There was a written exercise, but we also had to do role play and team exercises, then had a short interview with a panel of 3.
My advice to you would be not to be shy and try and show your personality.
Put yourself forward if a volunteer is needed for something, talk confidently about yourself, help others if you have to works as a group etc. Try to stand out.
If there is an individual interview, talk about the skills you have e.g.
Friendly and work well with others
Clear communicator
Well organised, good time keeper
Customer service experience
Cash handling experience
Stocktaking experience
And give examples of when you used these skills. If you do not have the work experience to talk about, give examples of school, clubs or any events you were involved in.
e.g I work well with other people and I would consider myself a good communicator.
An example of this would have been at school when I was part of a team that organised a fundrasing event. I was responsible for (blah blah blah) and in order to do this I had to carry out what the group leader told me to and give clear instructions to others.
I really enjoyed this experience and we raised £5,000.
You should prepare these examples in advance, maybe practice with a friend.