Is it safe to install Sierra with Apple Store blocked? I have tried to install El Capitan to the Mid 2011 İMac with 10.7.2 (Or maybe 10.7.5) and then it gave me do not cross icon at boot and it is useless now. I downloaded install os dmg file to my own drive and then added it to the applications 

When I asked to my teacher why it didn't work. they claimed the government has blocked the App Store and it will never boot again.

What I want to do is. Trying to install High Sierra without internet connection so this won't happen again. Is it possible to do it? I can disconnect Ethernet but I am fearing the WI-FI reconnecting to the school WI-FI even if I delete the password and yet worse, The operating system refusing to work without authenication from App Store after offline install or maybe even breaking itself when I connect to the internet.

Is this safe? I already got Time Machine backup in the the Mid 2011 İMac I want to update and I want to use the newer applications again.
I am providing the whole solution as it might help other too. This was executed by myself a couple of months ago. As you will have a macOS High Sierra offically from App Store also, it is safe to run the following option. 

First, you need one reference Macbook Pro or Mac mini whatever you can get. 

From the reference machine, download macOS High Sierra from the 
link below and follow the first step of clean install macOS Sierra on your USB:

https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?ls=1&amp;mt=12

That link should open the MAS and take you to the macOS High Sierra download page.

Clean Install macOS Sierra on your startup disk


Create a bootable flash drive with macOS Sierra.
First, make sure your USB flash drive is formatted and named as “Untitled”.

•  To format a USB drive launch Disk Utility (Finder -> /Applications/Utilities/Disk Utility).

•  Select your USB drive under External.

•  Click the Erase tab at the top of the Disk Utility window. Select Mac OS Extended (Journaled) from the format list. Then, click the Erase button. When the process is complete, click Done, and close the Disk Utility window.

•  If your drive is named something else, you need to rename it (Open Finder -> right click on the USB drive and choose Rename).
Now, your USB flash drive is ready.

•  Launch Terminal 

•  Enter the following text into Terminal:

sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\ Sierra.app –nointeraction


•  Hit the Enter key.

•  Terminal will ask you for an admin password (the one you use to log in). It won’t display characters when you type, so just do it. Hit Return. You’ll see that Terminal erases your drive. Then, Terminal will copy the installer file to your disk. This will take a few minutes. When the process is complete, the Terminal window will report "Done." 
You can quit Terminal and your bootable macOS Sierra installer USB drive is ready for use.


On your main Apple device

Plug in the bootable macOS Sierra installer USB drive to your Mac.


Restart your Mac while holding the Option or Cmd+R. Make sure to keep it pressed!
When Mac restarts, you’ll see the macOS Startup Manager with a list of bootable devices that your Mac can start up from.
Use the arrow keys to choose your USB drive with macOS Sierra installer. Press Enter.
Select Disk Utility.
In the window, at the top of the left bar, select your Mac’s Main Drive (it looks like a MacintoshHD).
Select the Erase tab located next to the First Aid button at the top.
Near the center of the window, select the Format drop-down list and select Mac OS X Extended (Journaled) type.
Click Erase (Note: this will erase all your data from your main drive!) and give the drive some time to format.
When it’s done, close Disk Utility and select “Install macOS” from the menu.
Select your main drive and install the new macOS Sierra 10.12.


Follow these Sierra install instructions and enjoy your newly clean-installed macOS Sierra.

Note: A bootable installer doesn't download macOS from the Internet, but it does require the Internet to get information specific to your Mac model, such as firmware updates. If you need to connect to a Wi-Fi network, use the Wi-Fi menu in the menu bar.