That particular error message 'in a class' can mean two things.
1. It might mean that you are currently enrolled in a free LearnToMod class; if your class has ended for the day, then you probably won't have access to LearnToMod's Minecraft server (though you will have access to the rest of the LearnToMod website content including the badges and simulator).
2. Or you have a copy of our book 'Modding for Kids For Dummies'. If you bought our book and set up an account with the access key on the back cover, then most likely your 2-3 trial access to our Minecraft server has expired. You should still have access to the rest of the LearnToMod website though.
3. Or you've run out of server and website access.
If you want to add more server access to your account, you can buy a LearnToMod subscription at our online store or elsewhere. After buying a subscription, we'll email you an access key. After you get the key, log in as usual and then on the LearnToMod homepage click the Account tab in the middle of the page. Then enter the new key into the days of access field. Or log in as usual and enter the new key when prompted. Your badges and mods will still be tied to your account.