The school issued MacBook Airs have a maximum cycle count of 1000, meaning they can be completely discharged and recharged 1000 times before being 'consumed'. A consumed battery doesn't mean a broken battery; it means you're likely to experience poor performance and reduced capacity. A battery cycle count of 500 on this particular MacBook model might roughly equate to a half consumed battery. You can dive into the battery life details at https://support.apple.com/en-us/ht201585.
The embedded video shows you how to check your device's battery cycle count. I hope it provides insight and helps you make an informed decision on the buy out option.