First of all, there is no such town/city/municipality as "North Edison".
This terminology was likely invented by real estate agents in order to distinguish the more desirable northern part of Edison from the somewhat less-desirable southern part of Edison. However, it is all the same town, with the same government, the same police force, and the same school system.
That being said, J.P. Stevens HS, which is the HS for the folks in the more northerly part of Edison, does have a better reputation than Edison HS, which services the southern portion of Edison.
Iselin is one of the many zipcodes that make up the township of Woodbridge. The "Iselin schools" are part of the Woodbridge Twp School District.
All of this being said, if I had a choice between sending my child to schools in Edison or in Woodbridge, it would be no contest--Edison by a country mile. Trust me, as a retired Woodbridge Twp educator, I experienced the steady decline in the quality of the school system that took place, beginning probably sometime in the late '80s.
As the inner-city element from Elizabeth and other cities to the north migrated to Avenel, Woodbridge, Colonia, Iselin, Fords, and other parts of the Woodbridge Twp, the crime rate in all of Woodbridge Twp began to grow, and the quality of its schools began to decline. Then, if you factor in the incredible nepotism and the promotion of unqualified (but politically-connected) people in the Woodbridge School District, you have the perfect storm for a reduction in standards and a decline in the quality of the schools.