Restarting your browser and/or your OS may help. The next thing to try is reinstalling the Osmo app that you are using to connect.
Please also try the username/password login (click the 'Sign in with a password' link in the Sign In pop-up window).
If that doesn't help, it may be a firewall problem. If your school is using a firewall, please allow all the playosmo.com subdomains ('*.playosmo.com'). If you cannot do that, please add the following domains to the whitelist:
You also have to unlock the network ports 80 and 443.
You may also encounter issues if you are using any MDM (Mobile Device Management) to manage the apps at school.
If none of the above solves the problem, please write us at firstname.lastname@example.org with the names of the device, the browser and/or the Osmo app that you're using to connect so we can provide further help.