In our science class at school we have allocated seats. I asked the teacher if I could change where I am because the people around me were distracting me. She said yes and now I’m moving to the back. But I’m worried the old people I used to sit with will think I’m a snob because I swapped seats. How do I tell them?
Too often, people worry about what others think. I've come to the conclusion that, unless I do something extreme, others don't really think about me all that much.
Moving seats in school happens all the time. Kids' vision changes, so they move to the front. They don't want to sit somewhere, so they move. Someone gets bullied, so they need to move. Someone needs their seat, so the teacher moves them. This is constant and expected.
That leads me to this: why do you feel you need to tell them? Unless they ask, this is really a non-issue. And generally they'd only ask if they wanted you sitting with them. I don't read that in the post, so I make the assumption that they're not good friends of yours.
That leads to this conclusion: a person that you aren't friends with changes seats in class. What is your reaction? Do you want them to move back? Do you care why they moved? I suspect the answer is "no". And if that's the case, the same applies to you.
Let's assume, for a moment, that these folks are your friends. Assuming you want to be honest with them, an answer like "I wanted to get more out of this class and we were having too much fun together so it was really hard for me to pay attention. It's not like I moved to a different city.." would generally suffice.