A few years ago in England (or at least Gloucestershire), this was a common synonym for playing truant. The phrase “I’m gonna doss science” meant that the child was not going to be in the science lesson illegally. However, more recently this word has mutated and now means a particularly easy lesson, i.e. one where you wouldn’t be any worse off if you didn’t turn up. For a lesson to be considered a doss, the work must be very easy and the teacher very lenient.
“Dossing around” in a lesson means messing around and playing up to the teacher, this can only be accomplished without punishment if the lesson is a doss.