Currently a signature (password entered into the signature panel at the bottom of a form) locks a communication permanently. The only way to enable a signer to update a communication, is to create a new version of the form (replicate your existing form and rename as V2), and send the new version to your contact who needs to make the changes. You would probably call the new version by the same name as the original, but include something like 'Version 2' to differentiate.
This solution is not entirely desirable, given you will have two different reports related to the one activity. One report for the initial form with all the students listed, and another report with the parent who has responded to Version 2.We are currently looking at how best to integrate 'update' and 'withdraw' into the Signmee workflow. We are discussing this in consultation with legal advisers, to ensure that we establish a workflow that is legally sound for all parties.As soon as the new 'update' and 'withdrawal' features are integrated, we will notify all Signmee users.