With the printers for the yearbook it would be a case of making sure as much as you can that they will keep your data safe. This will mean asking the printers for their data privacy statement for example they will not sell the photos onwards or reuse in other non related print material etc most big publisher will have a data privacy policy however some of the small local companies may not.
Also I would say it is about ensuring that you have taken all the necessary steps to safeguard the data (photos) like do not supply on a non-encrypted memory, if you are uploading the photos is the site secure and password protected etc