Changing medical permissions to opt-out would let parents retain their ability to choose who may see their child/children's medical information, while helping to mitigate allergies and conditions being entered into the system with no permissions granted at all.
Currently, many of our student's medical contact cards visible to staff are incomplete to a point that it is difficult to ensure that students will be properly protected while on trips or off school grounds (and in some cases, such as with severe allergies, while at the school as well).