I am also using my cell phone as a modem to connect to the internet. It is called teathering.
Some ways to make sure you do not lose much of your 5G cap is to make sure no video downloads and music are downloaded or streamed live. Make sure the wi-fi is disconnected after each session. Do not let it sit idle still connected to the internet.
The class sessions should be fine but if all three children will be sharing the 5GB limit. then it will be used up quickly depending if they watch or download anything of major size.
What I done for myself and my children was to buy them smartphones with the teathering option which gives them 5GB each per month but it costs more each month to have this option. That way each one has their own 5GB limit per month.
This is just a suggestion.
Yes you can block pictures from downloading by disabling the ActiveX control for your internet connections. You can do this by going to the control panel and opening Internet Options and Advanced Tab to show all the options for Active X controls.
Also when your emails have come into your mailbox, you can disconnect and view the files offline but you will not be able to send emails unless the connection is connected to the internet
You can also put a disk quota on the hard drive for each user which will allow them to download and save so much data to the hard drive.