According to Greatschools.net, Eagle River High is tops with 10 out of 10.
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Parent Reviews of Eagle River High School - Eagle River, Alaska - AK
It is a new school, about 3 or 4 years old. My kids are grown, but my neighbors' kids go there, and they all rave about it. I had one child in Chugiak, before ER was built. He was into sports, and happy there, but I would choose ER over Chugiak any day. Don't know much about the south Anchorage schools, but they do have a lower greatschools rating.