I think you ask a great question and on this forum I am sure you have gotten and will continue to get sound advice and suggestions on how to manage your time. There are plenty of self-help books like those mentioned above, but it takes time to read, implement the strategy and develop the habits.
The short answer is only you can find the best way for you to best manage your time. There is no one answer that fits everyone because we are all different. There are only 24 hours in a day so overloading yourself past a limit with reasonable sleep is not wise.
I will give you four different examples from one family: me and my three boys (the oldest is a second year Cadet at USAFA, the middle is a high school junior and the youngest is a high school freshman).
I liked to stay busy. My day was fairly structured in that I went to school, did what work I could for the evening during free time and socialized, then had sports practice after school and did home work in the evening. I usually had time for at least 5 to 6 hours of sleep which was plenty. However, I grew up in a different time (fewer games and distractions), in a small town (not much to do) and in a small school (there were 35 in my class).
My oldest set a track in junior high aiming to get an appointment to USAFA or USNA. He did this on his own. He really was forced to manage his time as he was involved in school activities, sports, church, CAP, etc. while taking a very strenuous course load in school. He loved learning but also loved computer games. He required less sleep than me and generally operated on about 5 hours a night. This is something that served him well at USAFA. He was not social so he didn't require social time on his calendar (would have driven me crazy but it suited him). He managed time by prioritizing needs to meet his goals. He graduated in a class of about a 1,000.
My middle child is very social. He loves video games. School is not near the priority to him that it is to my other two boys. He also plays sports, but struggles to find time to get school work done, practice sports, work a part-time job and socialize. He recently quit the part-time job during sports season to give him more time to focus on school and sports. He decided he wanted to keep his social life more than earn money (we will see how this works out when he has no money for his social life). He also doesn't sleep more than 5 to 6 hours a night. He doesn't make good use of his free time in school.
My youngest is the best time manager of the group. He is very involved in school, extra-curricular activities and likes gaming. He has a small social group, but they are close and do a lot together. He works on school work during free time in school. He will work on home work at lunch and in the car so he can have more time to do what he likes when he is at home. He has the discipline to work as needed. He likes to sleep and requires 8 hours a night.
None of us utilized a schedule or calendar to get things done. You will know if you over-schedule and then it is up to you to make the decisions on what to give up.