Go to Reporting > Custom Report, then New Report.
On the right side you will see an area you can select Contacts 1 and 2 (for student contacts).
Then Run Report at the base of the page. You will get a list of all students and their contacts, with a column for email address.
If you click on the column header for Contact 1 email, it will filter the report to show blank records first (those with no email). You can enter email addresses for those contacts on the student profile. You may also need to repeat this process for Contact 2 email column.
This will enable you to have full contact details should you need to email all parents in an emergency (e.g. school closure). You can also check mobile numbers in the same way, for sending SMS via Comms.