The short answer is you probably do not need to, at least not at this point.
A slightly longer answer is that it is prudent to document your teaching program with an independent tool such as an achievement test.
There have been legal challenges to Amish and Mennonite schools in the past, and voluntary use of achievement tests played a role in resolving conflict in several instances.
See this link for an article documenting a study using achievement tests with a number of Amish schools. This study was introduced as evidence in favor of Amish schools in Wisconsin v. Yoder. Note: The author believes the study was "marred by selection bias", but that does not negate the fact that achievement tests were considered useful in the study.
Another article refers to the continued use of achievement tests, particularly in Iowa and northern Indiana, out of concern for public perception.
Do we base our decisions on fear of public perception? No.
However, we are admonished in Colossians 4:5 to "Walk in wisdom toward them that are without, redeeming the time." That suggests to me that we be aware of the impression we leave with our neighbors and surrounding communities.
Obviously, Catforms stands to benefit financially if more schools use achievement tests, so we want to act with integrity in how we promote testing. There are many good reasons to use achievement tests and most of those reasons have more immediate benefits than legal protection.
Our position? There are many benefits in using achievement tests. Having independent test documentation for potential legal challenges is one of those benefits.