On the PC, once signed in to the network, open Google Chrome, and go to Gmail.com. You can do exactly the same on the iPad but we recommend using the dedicated 'Gmail' app, available from the app store, if it isn't already on your iPad.
Hit the Sign In button
On the iPad, you will see the options above, to 'Add account'. Even though your school email has @haslingdenhigh.com, it is a Google account, so select Google
On the PC, you will be invited to sign straight in. Use your school Gmail address provided to you, then click the Next button.
This time you will be automatically signed into Gmail. Accept prompts for notifications, if you like getting notifications when you get new emails.
You will also be asked weather you'd like to enable any new features in the current version of Gmail. This is totally your choice, and will likely depend on your views about privacy vs a more personal and 'tailored to you' experience.
You will find that you will now be able to open Google Chrome.....
.... and any other Google apps, and be already signed in or to sign in simply selecting the account first.