A journalistic essay combines the elements of journalism (like using factual evidence, interviews, current events, etc.) with the elements of a personal story. Keeping that in mind, you should consider two things: yourself and your audience.
Writers are often very successful writing to an audience of their peers. Naturally, when something is meaningful and relevant to you personally, it is likely meaningful and relevant to others who are in similar places in their own lives. Therefore, I imagine any topic that is currently interesting you will probably be interesting to others. You can make this as local or as global as you wish. Remember, journalism is done even as locally as within individual high schools. Personally, I encourage you to find a topic/issue that you and your friends are interested in.
I'll give you some generic ideas that could easily be written about more specifically and more personally: technology, sports, current peer-pressure, bullying, cell-phones, attitudes in the classroom, people who inspire you (as a teenager), etc.
Whatever you choose to write about, be sure to consider it both factually and personally. This means, pick a topic that has evidence from research, but something that you are personally affected by. Good luck.