The summer before high school I travelled to India and did some service there, can I write that on my Common App?
No; I would advise against it.
Yes it is.
It definitely is. You were in 9th grade as soon as you ‘graduated’ from 8th grade. You can refer to it as summer of 9th grade (or: summer, 9). If you are a senior this coming school year, right now is your 12th grade summer.
You should write it down, for sure.