The average height really has gone up. I was almost the shortest in my year in Year 8, and I haven't grown since. 5"1 isn't short for Year 8, yet I was still short compared to anyone else. Personally I think some kids will just decide that the shorter people in each year are short even if they aren't particularly.
Try not to listen to them Spicy, those in years below are probably jealous that you're in 6th form. It's likely they're not going to achieve much if they'll stoop so low so as to pick on 6th formers. People are going around saying that, "It's only 5C's to get into 6th form," but if it was so easy, more than about half of candidates would be getting it. These people probably aren't going to; they could be the tallest in their year but it won't get them GCSE's and A Levels. There are things much more important than height