i think it is reasonable to expect the school to ask but i dont really see it as a big problem
If they did take a dog [ having completed the risk assessments for activity breaks] then they need to have mentioned the dog in the risk assessment and have done things to control for this- seriously Health and safety gone mad innit
Given they can personally be sued for manslaughter if the trip goes wrong they treat these very seriously
All risk should be mentioned to parents if only to cover your arse from the over sensitive souls suing you
If you really want to kick up a fuss ask to see the risk assessment for the trip.
FWIW they were an absolute pain in the arse to do you had to think of everything possible that could happen and I do think the school should have mentioned it to you
Personally I think there is no more risk from a dog than from say abseiling but i would expect to tell parents of either and they would be on my RA