Timeline of changes in students in my year group

Hello everyone;
So, since I started school at ST. Andrews International School around the last fraction of year 7, people in my year group had since undergo numerous changes, all of which are in the form of new students joining and old students leaving, affecting the population of the year group.
Year 8: Most people that I've started to know since the last 2-3 weeks of year 7 were still around, some, if not most, are still with me until present. The population stables around mid to higher-50s
Year 9: This is where the big change takes place. Not many students had left, probably less than 5; but the number of new students joining is numerous; about 7 to 9. This pushes the population of the year group to around lower-60s, and yet again, some, if not most old students are still around.
Year 10: Like year 9 there has been numerous changes. After year 9 I think there is barely any student that had left, but from my memory only 1 student that is known to had left. But still the number of new students joining is quite huge, around more than 5; the number could go up as high as that of year 9. This pushes the population up even more, I think it is somewhere near 70; but since about 3-4 existing students are known to had left, this brings the number of students down to about mid-60s.
So, as you can see, the population of my year group experienced the most rapid growth during year 9 and 10. I think my year group is now the most populous year group in the school.