What you said
Go ahead with the cookery. Can you ask a colleague to have the "worksheet candidates" in their class? Or the library? You may find they decide to join you when they realise they are excusing themselves from the fun.
Really big this up to them and their form teacher. Send messages to be read in registration time about bringing the money. Get the form teacher on board. They are teenagers, so won't show the same excitement you feel. But keep with it.
The expert view
Go ahead with this, but consider that this kind of refusal may be financial. There are many kids for whom pound;1 is a lot if they are from homes where parents hide when the letters fall through the postbox because they're bills that can't be paid. In this case, children are often anxious about any activity that involves a contribution.
Also, the school isn't allowed to ask parents to pay extra for activities, unless this has been agreed by prior consent. The school can ask for contributions, but must be explicit that these are voluntary - so that schools can avoid making poor parents feel inferior or obliged to have their children excluded from activities.
I'd advise you to do the activity, but let pupils know it's voluntary. If there's a shortfall, I suggest that you make it up or ask the school to cough up pound;20 or so. It would be wrong to make the non-payers sit out the fun while they watch the others bake and gorge; indeed, avoiding that kind of exclusion was one of the reasons our society decided to have state education.
Of course, it may be that they are just being obstructive, but this isn't a battle you should fight with them. Do the activity in the spirit of reward and education and you will reap the benefits of that generosity.
Tom Bennett is author of `The Behaviour Guru' and `Not Quite a Teacher'. See: http:behaviourguru.blogspot.com
Post your questions at www.tes.co.ukbehaviour.