I took a job in a prep school once (i'm secondary). To be honest i was bored stiff and missed the stimulation and challenge of gcse and a level classes. I also found it weird how they cared more about how many 'matches' you supervised than how much academic progress your classes made. I felt like a childminder for the wealthy and left after 1 year. I did struggle a little to get back into state - they are generally quite suspicious and suspect you might not be able to hack it. I would be very careful not to stay too long as without exam class experience it would be v hard to get back into the state sector. Oh and you'd probably have to work saturdays, longer contact hours during the week, and get paid less with no career progression. I would advise 'don't do it' but i'm sure plenty of people will say the easy life of a prep school is a dream!