Before you start deleting, make sure you're really ready to let go. If you're a teacher, you'll want to archive your classroom instead of deleting it, just in case you need to access those old materials again. And if you're a student, you might want to download any important files before bidding adieu to the classroom.
Now, onto the fun part - actually deleting Google Classroom! If you're using a computer, simply sign in to your Google account, click on the classroom you want to delete, and hit the three dots in the top right corner. From there, select 'Delete' and confirm that you're sure (you're sure, right?)