I suspect site may have been hacked. Hw do I change/reset system password.

Also how do I lock classmate profile and missing classmates so password is required to join site.

You can edit (or remove) the system password by going to Edit Site Pages. The options are at the bottom.

Password protecting missing classmates could be done, but it's not really a workable solution. You'd have to add a password to each one, and then give them out when asked by a Contact Us message or phone call/email (assuming they have some additional way to contact you).

Fortunately, there's a handy way to ensure a new member is valid. On the Preferences page, there's an option called Admin Verify which (when turned on) still allows anyone to join, fill in some contact data, profile data, and even change the name. But... they still can't see any restricted pages (or profiles), and can't actually do anything - until you verify that they are who they claim to be and click the Verify box in their profile.

Sending an email to the email address they entered, or calling them at the phone number they entered, and asking a question that only someone from your class would know works quite well. And if someone tries to hijack any missing name and can't answer your questions (or be contacted using the info they entered), Admin Verify gives you the option to instantly restore that ID to it's original state.