To prevent security issues, you must use the activation code to connect the iPads.
To connect an iPad to your myOsmo account using your activation code, please open any Osmo app and tap the myOsmo button (or a profile avatar button) in the top right corner of the home screen. Then tap 'Log-in', and tap 'School Login' in the pop-up window to connect with your activation code.
The school login doesn't log into the browser, so the students can't access the teacher's account.
If a teacher uses the email login to connect the iPads, then yes, her students can access the account and delete it. But she can also sign out from the browser to prevent that (and that won't log out the Osmo apps).
Please also see our PDF guide to myOsmo here: