Who did the study? Where is it published? Define “Modern Orthodox”? In many cases, a “modern orthodox” high school is one in which the majority of the families are not strictly Shomer Shabbos and Shomer Kashruth to begin with. If 50% are Shomer Shabbos after high school in spite of 40% of the parents having been Shomer Shabbos, the school is doing quite well.
Was he talking about a school of “penguins” who are considered “modern” by virtue of the fact they celebrate Israeli Independence Day and don’t learn Yiddish? Or was he talking about a school that is serving a wider community and in which only a minority of the familes are frum.