Given how many people have mobile phones, it would not be particularly difficult for a child to obtain phone numbers. They could easily get the student's number from their contacts, and with a little effort, gain access to their contacts using a number of methods. I think the risk of bullying is important, as it has proven difficult to prevent. I therefore suggest that we do our best not to make it easier.
However, let's consider some ways the access of these accounts could be abused by others who are not schoolmates. This is yet another source of data, and one that companies would not mind mining. Do we want our children to have their primary school grades analyzed or leaked? I think we can all agree the answer is no. There are many parents who obsessively check the grades of their children, but some of them* would not mind seeing the grades of other children for comparison. They could use this insecurity as an entrance. If we want to overthink this, there is probably a lot of personal information in this that could be used to socially engineer the child, too.
Children do not have much data on them. Their schoolwork can be a very personal thing. Some may divulge it to others, which is entirely their right, but others do not want their friends or anyone else to know all the details. I believe it is extremely important that it remain private to them and their parents. The worst-case scenario with a leaked phone number is irritating calls. This is certainly a thing to be avoided, but I can think of worse things that could be done with leaked educational data.
*Parents spying on students' grades: I know parents who do this, usually by "casually" asking students increasingly leading questions. It is not that many of them, but one is already too many and there is more than one.
Biting the hand that feeds IT
