Well...
If you're studying at school:
grapes,
berries,
water,
juice (cranberry, apple, orange, etc.)
try crackers with peanut butter
slice up apples, peaches, pears, etc. and put them in a container
nuts are good
Anything that's healthy.
When you're at home studying you can eat all of that stuff too.
Entering high school will be a great experience and it's great to see you're interested in keeping yourself healthy!
Honestly, you won't be doing a whole lot of studying unless you take all AP (advanced-placement) courses.
I take all AP and I do study a lot.
My classes are interesting and I really like eating grapes. So my biggest suggestion is grapes. You can put them in your purse/lunch bag/backpack and they don't ever get too warm. Just do be careful when toting food because they can get mashed.
Good luck, and you are totally on the right healthy track!!