Why does a students contact email address have a line through it when I choose them to Comms to send an email?

It is very likely you have the "Allow Contact By Email" field set to No.

To change this go to the student's profile, click their contact and click the black cog icon next to their name.  Simply change this field to Yes and save. 

Now when you go back into the Comms area you will be able to send your email.