Remember that the formula for an arithmetic sequence is
Where a is the term you want to find, d is the common difference, n is the number of the step you want to annalyse, and c is the value of the first term of the sequence.
In grade one (step zero of the sequence) we have 130 students, so
If we take the number of students in grade two (step one) and substract the 130 students in grade one, we can find out what the common difference is
The formula of the arithmetic sequence would be
Notice it checks out with the number of students in grade three (step 2)
To know the total amount of students in the school we will have to add every single step of the sequence (from 0 to 11)
In other words,
So, the total amount of students in the school , if the school goes to grade 12, would be 6840 students.