I don't see a problem with a lack of wifi in schools (with an exception for College/University, and only in designated areas), but not because of any supposed medical reasons.
What reason would any grade-school kid need wifi access for, anyways? What device would a grade school kid have that would even have use for wifi? A laptop? Why would a grade school kid have one? Even if they did, what use would the make of it in school (on a regular enough basis to warrant a wifi network)? A wifi enabled cell phone? You don't need wifi to make a call or send a text, and the phone should be off during class anyways.