Clearly, implied consent presents no evidence that consent has taken place. Instead, a notice from school may well be buried in some backpack or lunchbox, only to be discovered long after the invasive survey has been given. Therefore, the only way in which to assure parents have consented to their child's participation in a research project is to obtain a parent's signature. I appreciate that written consent may present administrative and cost burdens for researchers. However well-intended the professionals involved in a child's life may be, teachers, school principals and researchers do not know more or care more about a child than a child's parent.
Parents are best suited to determine which activities are appropriate for their own children.
And parents should have the freedom and discretion to say yes or no to intrusive surveys involving their own children.