Newsletters, booking confirmations, reminders and follow-up emails are sent by the system with your school email address in the "reply-to" field. That means if your customer replies to your newsletter or booking confirmation, it will be sent direct from them to you.
But if someone marks an email as Spam (using Report, 'Move to Junk' or similar), this can generate feedback called a complaint from the recipient's mail server. Spam reports have a negative effect on the reputation of the sender, in this case Flight School Booking. Mail servers aiming to keep spam from their customer's accounts will use reputation as one of a number of scores feeding into a set of spam detection rules.
Bounce reports also feed into the system. When email is sent to an address that is invalid, the system is notified.
The system reacts by unsubscribing users from future emails. This means if one of your customers reports your newsletter as spam (and if their mail system sends feedback), they will no longer receive your newsletters, booking confirmations, reminders, follow-up emails and so on. They will still receive password resets and billing notifications. It's likely anyone reporting spam from the system has had a trial flight in the past and no longer wants to receive emails from you. But if real customers report spam by mistake, they can re-subscribe from their Settings tab.