Heisachi for high school entrance exams?

My family is moving from the US to Tokyo in December. I'm completely fluent in Japanese, but I still need to know what my heisachi (z-score) is to decide which schools I should go to. But since I live in the US I've never been to juku (cramming school? not sure what it's called in english), and I've never taken the tests they give you that tell you what your score is. Is there any way I can take a test online for free or something like that to find out what my score is? (also I'm not asking for how to solve the z-score, just how to take tests that will tell me my score.)
Thank you in advance!
Kana

Hensachi is a Japanese standardized test similar to "SAT(Scholastic Assessment Test)" in the US. Therefore you can't be taken this test outside JP and there is no formal test on this(each testing organizers measure in their own way).

Transferring US to JP can be consider either you study at an American school in JP, or, an International school which is taught in English, or, a Japanese high school which strongly emphasis both English and Japanese language studies such as ICU High School in Tokyo.
You can't be measures your Hensachi score while you have never studies secondary school studies in Japanese.

First of all, it is not heisachi but heNsachi(偏差値).
You need to take exams that are open to students.
Some 塾 offers tests for free online (though I cannot tell how accurate they are.)

http://www.ehle.ac.jp/ybk/net-mosi/
just for example.

You should be able to find more information if you search with some of 偏差値, 模試, 公開, テスト as key words.

Kanaさん、こんにちは。

大学受験でしょうか、高校受験でしょうか。いずれにしても、現在、受講しておられる通信教育の会社や日本語学校に相談されるといいと思います。

海外で受けられる模擬テストはたくさんありますよ。受ければ、あなたの偏差値もわかります。

https://www.google.co.jp/?gfe_rd=cr&ei=UmcGVeyRN9CF4AKKsoHIBw&gws_rd=s...

I don't know if you're aiming for college or high school, but you can take prep tests while being overseas. If you take the test, you will get your results along with your hensachi. You can also discuss details with whatever Japanese school you're currently studying with.

Don't worry about it, study hard and enjoy life!

Thank you guys so much for the answrs! The links you left were very helpful. (Also sorry I said "heisachi" that was autocorrect on my phone XD)
Kana