It might seem ridiculous having a group interview with 'group tasks' for a summer job, but from their perspective, they probably hire a load of students at the start of summer when school finishes. By having a group interview, they can hire a lot of people in one go, not to mention, group interviews are the best way to see how you perform with others and interact with people.
The most important thing is that if you ever get asked a question, answer it to the whole group, not just return it at that interviewer, things like that will help you.
On another note, I live in an affluent London commuter town, where Waitrose is our main supermarket. Most of the workers there are from local private schools... it does have a somewhat different image with its staff to other supermarkets, but at the sametime, as a lot of them are young, I often see them just chatting while they are supposed to be working. It's also quite funny as you see a lot of well off 16-18 year old girls who work there who come all dolled up to stack shelves in their uniforms!