I'm not quite sure how to express this, except in Javascript, which is how the rating calculator at http://istariareference.com/arc.asp shows it. The game calculates it the same way but a different programming language. But this is the javascript version.
Code:
sum = Math.pow(theform.school1.value,2);
sum += Math.pow(theform.school2.value/2,2);
sum += Math.pow(theform.school3.value/2,2);
sum += Math.pow(theform.school4.value/2,2);
sum += Math.pow(theform.school5.value/2,2);
sum += Math.pow(theform.school6.value/2,2);
sum += Math.pow(theform.school7.value/2,2);
sum += Math.pow(theform.school8.value/2,2);
sum += Math.pow(theform.school9.value/2,2);
sum += Math.pow(theform.school10.value/2,2);
sum += Math.pow(theform.school11.value/2,2);
sum += Math.pow(theform.school12.value/2,2);
sum += Math.pow(theform.school13.value/2,2);
sum += Math.pow(theform.school14.value/2,2);
sum += Math.pow(theform.school15.value/2,2);
sum += Math.pow(theform.school16.value/2,2);
sum += Math.pow(theform.school17.value/2,2);
sum += Math.pow(theform.school18.value/2,2);
sum += Math.pow(theform.school19.value/2,2);
sum += Math.pow(theform.school20.value/2,2);
sum += Math.pow(theform.school21.value/2,2);
sum += Math.pow(theform.school22.value/2,2);
sum += Math.pow(theform.school23.value/2,2);
sum += Math.pow(theform.school24.value/2,2);
sum += Math.pow(theform.school25.value/2,2);
sum += Math.pow(theform.school26.value/2,2);
sum += Math.pow(theform.school27.value/2,2);
sum += Math.pow(theform.school28.value/2,2);
theform.rating.value = Math.floor(Math.sqrt(sum));
The first line (school1) is your Current School. All the remaining lines are school you have, but not currently joined in.
So Current School is the level value with a power of 2. So if your current school is level 20, a power of 2 means multiply itself 2 times (20 * 20 = 400). So value,2 means power of 2.
All remaining schools (that are not your currently joined school) will be divided by 2 first, then power of 2.
So if School 2 is 20, divide by 2 = 10. 10 with a power of 2 (10 * 10 = 100). value/2,2 means divide the value by 2 first, then do power of 2.
After all of your schools are added up, then it takes the square root of the total sum.
Whatever your current school joined is, plays a big part on the calculation (because current school is not divided by 2 first, like the others).