Sunset Shimmer is in 12th grade
Fluttershy: Maybe it was different at your old school, but at C.H.S., everybody sticks to their own kind. One thing that they do have in common is that they know Sunset Shimmer is gonna rule the school until we graduate.
They are going to graduate at the same time as sunset shimmer thus, they are also in 12th grade.
12th grade is 17-18 years old
Fluttershy is one year older than Pinkie Pie
Fluttershy is 18
Pinkie Pie is 17
The rest of the Mane six are 17-18