To prevent security issues, you must use your activation code to connect the devices to your account.
You can find your activation code on your myOsmo account home page. It will only be displayed if you have a school account, so if you don't see it, you may need to update your account information.
To do that, click on 'Account' on the left of your myOsmo home page and select 'At School' as the location where you use your Osmo.
To connect a new device to your myOsmo account, please open any Osmo app and follow the instructions to log into your account.
That doesn't log you into the browser, so the students can't access the teacher's account.
If a teacher uses the email login to connect the devices, then yes, their students can access the teacher's account and delete it. However, the teacher 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.