Forwarding Calls
From your classroom/office phone:
Pick up the phone. When you hear dial tone, dial *64 then 7 followed by the 7 digit phone number.

If you have non-218 number; *64 then 7 followed by 1 then area code followed by 7 digit phone number.

To cancel, pick up the phone again and dial *73.

Remotely Checking Voicemail
If you opt not to forward calls to your extension, you can easily check voice messages from any phone. Just dial your school number, then your extension. When your greeting starts, hit the " * " key, then your 4 digit passcode.

Google Classroom app has an annotations tool, and students can write or draw on the PDF or Google doc. See this and other Google Classroom tips:

Leveraging the Amazing Mobile Features of Google Classroom

Use Google Chat as a communications tool.

If you have students with limited internet access, Google Classroom and the entire Google Suite can be used offline. Meaning -- Students can go to an area with internet access and access Google Classroom and all of the assignments will update, and a student can work on them offline. When they're connected to the internet their work will transfer. Note that if you embed a link or video those will not work without internet access. Students will be able to edit docs, spreadsheet, and slides.