Copy this HTML script. Go to Email Settings > Signatures > Add. Paste in the script and adjust it with the correct values.

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<span style="color:#820024">First name Last Name</span> | Position at School<br />

<span style="color:#820024">School Name</span><br />

School Street Address, School Suburb State Postcode<br />

Tel: 02 1234 1234 | Fax:&nbsp;02 1234 1235 | Direct: 02 1234 1236<br />

Email:&nbsp;<a href="mailto:firstnamelatsname@schooldomain.nsw.edu.au">firstname.lastname@schooldomain.nsw.edu.au</a>&nbsp;|&nbsp;<a href="http://www.schooldomain.nsw.edu.au/">www.schooldomain.nsw.edu.au</a>

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<p>Disclaimer: The content of this e-mail is intended solely for the use of the individual or entity to whom it is addressed. If you have received this communication in error, be aware that forwarding it, copying it, or in any way disclosing its content to any other person, is strictly prohibited. If you have received this communication in error, please notify the author by replying to this e-mail immediately.</p>

This is a sample of the above HTML email signature.