I have to say I don't love this kind of activity as a selection method, in fact 'teamwork' activities are many people's worst nightmares and people with far more experience than these 6th formers might struggle to show themselves to best advantage in the 'build a raft from spaghetti and string' type scenario - you tend to get the loudest/most confident person taking over the whole group, which is fine if you specifically want to recruit that kind of person but anyone a bit shy will struggle - I'd much prefer to do individual or one on one interviews if you can manage it, it can be done in a very informal, supportive way and be a great opportunity for interview practice for them if nothing else.
But if you really want a group activity, the things that work better IMO are more related to the actual world of work (unless your line of work is in fact tent design/assembly in which case I apologise 
), and which are designed to give everyone a role to play rather than being more free-form. So something along the lines of, set up a loosely realistic scenario they have to work together as a group on, and give everyone a 'piece of the puzzle' to solve the problem.
E.g. ask them to imagine they work for a company which has a pot of money to spend on a project (make it something related to your line of work). You can give them short descriptions of 3 options or proposals to decide between, and handout to each person a card or document with some of the information the group needs to make the decision. (person A has the financial data, B has information from suppliers about equipment/facilities, C has the views of staff, D has the latest customer survey, E has the opinion of senior management etc.). You can make the information/scenario as easy or hard to understand as you want to suit their skills/knowledge/experience. They then need to discuss what information and viewpoint each has, listen to one another, assimilate the information, exercise persuasion skills, prioritisation etc. to ensure their view is heard and come up with a sensible answer within a defined time period (usually I set it up so none of the proposals/scenarios is the right or wrong answer per se, the point of the exercise is how they work as a group to form a decision not what decision they actually make).
You can follow this up with getting them to present back to whoever is assessing them on their chosen proposal and why its the best option, or if written skills are more important in the role they'll be doing than public speaking/presentations, get each to write up a short item on the proposal, for a newsletter or the local press or whatever fits the scenario. This gives anyone who has perhaps been a bit quiet in the group setting a chance to express themselves more fully which I like...