Can Mail warn when emailing certain addresses?
Yes, you can have Mail put certain addresses in the to field in black and all others in red.
I use this to have Mail flag email that's going outside my organization.  

Do this by going to Preferences > Composing.
Then under the "Addressing" header check "Mark addresses not ending with [ ]."  Inside the box put ".nameofyourschool.edu".
Then every email address destined within your school will be black, and all others will be red.
Just the thing to ensure FERPA compliance.