An exercise is saved once you press the 'Finish' button. On your results page, you can see all exercises you completed. Everything you do in your account is saved in a log file. Results can't disappear or be removed from your results page or from the log file.
In English Essentials, there is a counter that counts the number of passes and fails for each skill. This counter is reset at the start of the new school year. Older results will not be included in the counter, but you can still see older results in your overview.