This is a really common question to receive in the prelim exams I have being told.
Because it is a very broad question I suggest researching the health of Year 11 students, what are their eating and physical activity patterns? Are they receiving the appropriate amounts of nutrients. Are year 11 students using drugs or sexually active? If so they may be at risk of STIs or other diseases.
These statistics are normally easy to find in many PDHPE textbooks, I use the Oxford PDHPE Application and Inquiry textbook and it has many articles about these behaviors in adolescents. You can always try the internet just be sure that it is reliable.
In regard to the actual assignment to develop an action plan you need to firstly establish long-term and short-term goals of your action plan, what do you want to achieve in a week, a month, a year and so on. These goals must take into account the target group's (Year 11) capabilities, environment and access to health services and support.
For example if someone wants to loose weight their short term goal may be to eat healthy for a week but long term they may want to lose 10 kilos.
When setting these goals follow what is known as the SMART principle.
Specific- goals that are clear and specific
Measurable- Provide opportunity to monitor progress and be able to identify when a goal has being reached.
Achievable- The goals must be achievable and take into barriers in implementing your action plan
Realistic- The goals much be challenging but still can be realistically achieved
Timed- There should be timelines set on the different goals you wish to achieve.
It is then a good idea to construct a timeline for when your different goals will be implemented, be sure to also set deadlines so that the action plan will continue to move forward.
I suggest looking at the Ottawa Charter, that is a very clear structure of an action plan which is used to improve personal health. If you follow the indicators in the Charter than you can build an effective plan.
Most of that was off the top of my head so I know that it was a bit broad, if you want me to go into more detail in a certain section just ask. Good luck