Did you agree to this target? Didn't you have any kind of meeting and discussion? I would ask the head to meet with you and explain your dilemma... but go with some achievable targets in mind. It does sound like they're very happy to have employed you as a general dogsbody... but they obviously think you're capable or the wouldn't ask you to do it. I'd stop panicking and try the most obvious route... a good old conversation! You could always ask if your target is maths related for Y6 could your time teaching them maths be protected and could you do your own lesson planning for those lessons... you might find the other Y6 teacher loves you for this... maybe they could cover you in whatever area you've been thrown while you teach the kids maths... All I'm saying is why not be a little proactive... discuss it with the other teachers first if you want to get a feel for how performance management works in the school. But don't wait in dread... tackle it before it does become stressful or a problem.