In my opinion, it sounds as if they simply do not like boxers. I had this occur with a few trainers in my past. So...if boxers are pointed out as not being up to their expections, then I would quickly go to another daycare that do not have the same opinion.
The trainers said that most boxers are stubborn and not teachable. I know this has nothing to do with the "boxer stare" question, but it did remind me of my trainer experience. Because the trainers were predisposed to not liking boxers, I truly beleive, I had a bad experience as the training continued. I cut the training off after several sessions, when I saw that Samson, my oldest boxer, was not being treated very gently and patiently.
My Delilah is a little on the agressive side, mainly with food. We work with her daily with this problem. She does have a "stare" when she is in one of those moods. My other two boxers do not have a "stare."
I would chalk this up to the person possibly not liking boxers, and everyone has their own likes/dislikes, and look for another doggy day care.