You could create a new role that just sets the few capabilities you mention to PREVENT, let's call it the 'Restrict student' role. Then, when you create the dummy account, you can give it the 'Restrict student' role globally, and that will mean the people logged in with the account you create can never do the things you are worried about, not matter what other roles they are given elsewhere in the site.
Roles and permissions
Help Please: Creating a basic role
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