It really shouldn't be a problem unless they were actually your students. Unless you go up to them and say, "Hi. I'm your dad's therapy client", they aren't going to know or really care who you are. Is it going to bother you to see them? If so, why? You also mentioned school functions. Again, how bothered are you going to be by seeing your therapist in his dad role?

One of my therapists happened to be my pastor years ago. I saw him constantly outside our sessions in his pastor role and we just kind of had two different modes with each other: the pastor/parishioner relationship and the therapist/client relationship. It really didn't seem to be any big deal. Probably helped that he was my pastor long before he was ever my therapist.

We all have to shift roles from environment to environment. I'm teacher in my school, but as soon as I step foot on my sons' campuses I take off my teacher hat and put on my mom hat. Your T needs to be able to do the same. He needs to be able to completely be dad at his kid's school. Will you be able to let him just be dad without feeling jealous or slighted in any way?