When faced with a heavy workload, it can be very daunting to approach your responsibilities without being too overwhelmed. Go through your work methodically and make sure to take breaks so that your brain can keep up steadily with the work. When faced with less daunting pieces of work, rather than a huge boulder, it is easier to tackle the problem.
Say you have 3 tasks to complete for your first extracurricular, 2 tasks for your second extracurricular, 2 assignments for school due on Friday, and 1 assignment for school due the following Monday. When breaking down your work and setting manageable time goals, you are allowing yourself reprieves. Say you have 4 hours to finish the first set of work and the 2 assignments from school take longer than the rest. Depending on the type of student you are, you can start on these assignments and alternate every 30 minutes so that you can take a break from each subject and start fresh every time. Then after you finish your first assignment, you can alternate between your second assignment and your first extracurricular task.
To figure out your manageable chunks, consider the following:
Due dates - are there assignments that you should be prioritizing?
Motivation - are your creative juices flowing right now? Would it be a better usage of your time to do something creative rather than a research/desk-job?
How are you feeling physically? Are you in the mood to do a task that could involve more physical activity or would you prefer to do something less physical?
Do you have any distractions around you that could impact your quality of work. If the assignment is formal, would it be better to wait to complete the assignment so that you can be in a less distracting environment?
Have fun and take your time. The best part of being an adolescent is the opportunity to make mistakes. <3