I hate my job at River Island

In any job, you won't expect to be best friends with your colleagues, so long as you can work together and be civil towards each other. Your colleagues will have different interests to you as they are of different ages, and it can feel alienating, but what you have to remember is that you are there to work and make money, not socialise. If you can make friends at work, it is a bonus but not the most important thing.

You are making money and getting a good discount, and it does provide good experience for future jobs. Retail isn't always the most exciting of work, I imagine you must have busy periods too. You won't be working there forever, just to save some money for things that you want to do later on. If you can stick it out, try to do so. If you really can't stand it, you could start to look for other jobs.

do less hours...go on your phone

I had the same problem with Debenhams (I lasted 6 weeks). I worked previous jobs and loved actually having work to do. But at Debenhams we were always overstaffed and virtually nobody came into the store, so everyone would desperately try to find something they could do to pass the time. I would recommend keeping at it for the experience/money, whilst trying to apply for other jobs; and possibly mention to your supervisor/manager that you feel like you're not being very productive.

I'd love to get paid to stare into space for 3-5 hours. I don't understand when my colleagues complain that there is "no work to do" because we are literally getting paid to do nothing.

If it's impacting your happiness or your education, leave. A part time job in retail simply isn't worth the stress.

I'm too afraid to ask for less hours, there are easily many other people who would happily take my job with the many hours without a complaint so I'm afraid i'll lose my job if i ask.