The proxy section should contain your proxy details. For most schools, entering "DEFAULT" into the Proxy field will work, meaning the other three fields can be left blank. If this does not work, you will need to enter the correct proxy details for your school.
Once the details have been entered, click the Save button at the top right of the page and then click the "Test" link. Cura will attempt to communicate with www.google.co.uk and www.tascsoftware.co.uk and will then inform you of the results. If both connections are successful, your proxy is set up correctly.
Click the cog button at the top right