You can take any type of classes related to food which are offered in the high school. Some classes may be regular in high school but some other may be part time class. You have to be a volunteer in the kitchen, whether it is school kitchen or local soup kitchen or even in the small catering business, these will give you experience in the kitchen.
How to get a job?
You should try to get summer jobs in the kitchen. You have to try to find the job working in the kitchen. It will give you a real world experience. You have to keep your resume ready. You have to keep contact with every school or baking or pastry chef class where you learned cooking. In the pastry chef career these experiences will help you. So you have to keep contact with the school every time. To get a pastry chef certificate or degree you have to complete the bachelor degree. You should have a formal degree or a certificate with training. It will give you correct knowledge about pastry chef and you will be able to succeed in the pastry chef field. Then you too try for a job, after getting a first job, you have to work in kitchen. So the first job in the pastry chef will not be so glamorous. But it will give you some sort of experience as pastry chef. You have to keep on trying for getting a new job. Once you get a first job it will be easy for you to continue with the first job until you get the better job of your choice.