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Hi @Anonymous,
In order to be able to use 2015 version of AutoCAD on macOS Sierra, you need to install an Update to AutoCAD 2015 form here:
https://knowledge.autodesk.com/support/autocad-for-mac/downloads/caas/downloads/content/autocad-2015-for-mac-update-4.html?v=2015
At the same time, as you have student subscription, you are eligible to use the latest version of AutoCAD - 2017, which is fully compatible with Sierra. If you don't see download link to 2017 version in your account management page, you can download it using Autodesk Virtual Agent:
http://autodesk.creativevirtual15.com/autodesk/bot.html?isJSEnabled=1&businessArea=Root.Front%20Desk&startContext=Root.Front%20Desk
https://knowledge.autodesk.com/community/screencast/9e086aca-5439-4336-9a1d-e33ff069d60b
You just need to use the link to Mac version:

Now you can install 2017 version and activate it with your student serial number. The product key for 2017 version is 777I1
Maxim