1. Google Classroom is a black box
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2. Google Classroom is not a course management system.
If you are looking for a place to set up units and prepare folders of resources ahead of time, Google Classroom is not for you; Google Drive is definitely for you, but not Google Classroom. Drive will allow you to set up files for units and plan resources ahead of time. If you want your students to have access to all of your files from day one you can share the folder with your students. You could share the folder on Classroom, but do not think of Classroom as a course website, but rather as a Facebook news feed or a Google stream.
3. Google Classroom is all about the here and now.
Google Classroom's Stream will make it easy for students to find exactly what they need for a particular moment in time. The teacher needs to update the Stream often so that the relevant information for class that day is at the top of the stream, ready to go. If you are in a one-to-one device environment, this can help simplify your day significantly.
This focus on the present can also cause issues. If, like me, you are planning on using Google Classroom daily you may find the format of the Stream frustrating. Throughout a unit I will provide information sheets and links that students need to access throughout the next couple of weeks. This information quickly gets pushed down the stream and ends up requiring scrolling and page refreshing (which has recently resulted in the page automatically scrolling all of the way to the top) to find exactly what a student needs. Students could "make a copy" of each information sheet, or copy links into their bookmarks, but in reality, this will almost never happen.
4. Assessment in Google Classroom will not work for everyone.
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The key issue I have is that evaluating the work when it is finished is tailored to a very specific form of assessment. The built in assessment functionality only lets you mark the assignment based on an overall average or point system. Our school system marks with a 4 category, 5 scale rubric. There is no way to set a rubric as a marking schema, and if you do not attach the rubric when you originally create the assignment (perhaps you are co-creating your rubric with the students) you are not able to edit the assignment and add the rubric with the option of giving it to every student.
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5. Google Classroom's usefulness is dependent on your use of Google Apps for Education.
If you find that you use a wide range of websites, online tools, and applications, you may find Google Classroom is just another site you need to maintain. But, if you use Google Docs as well as the many other Google Apps, Google Classroom is the place for you. The ease of sharing Docs assignment sheets with each of your students and the ability to easily track who has submitted what and when by looking at the submission history, makes this a great tool. It also makes it very easy to share course content, whether it be a youtube video, a link to a webpage, or a variety of handouts and slideshows. Students will have no good excuse for not finding everything they need.