Under 5s do not need to wear a face covering on public transport. Also, anyone who cannot put on, wear or remove a face covering without it causing severe distress, or because of a physical or mental illness, impairment or disability, are also exempt.
If you are exempt from, the easiest way to let your driver know, is to show them your Thistle Assistance Card. You can request your card at http://www.thistleassistance.com or search 'Thistle Card' on the App Store and download the app. Simply show the driver your card and board the bus as normal.