One reason students don't do well in school is that they fail to manage their time. They have vague ideas about when to study and how much time to devote to study. Study time must be scheduled and honored. Get a time management worksheet, available on line. Just google “time management worksheet,” and print up a copy. There are several of these sheets offered free; pick one that you think will work best for you. Fill it out. Several print books have time sheets included. I love the one in "Becoming A Master Student" by Dave Ellis. The book is worth buying, reading and studying.
Back to time management sheets. Where do you spend your time? Put aside time for cleaning house, sleeping, shopping, and work. Don't forget class time. Be aware of hidden time pockets. Often things on your list take longer than expected. People writing out these charts will factor in cooking and eating time, but forget clean-up time. They might factor in time spent at work, but forget the time it takes to get to and from work. What about unexpected phone calls? It also takes time to get to and from an on-ground class. Factor in, at least an hour a day to study for each class.
You might need more than just a tine management sheet. I have a day planner to pencil in appointments. I also keep to do lists. The to do list might be daily or weekly. Everyone's schedule is different, so all of our time planners will look different. Some people like to keep everything in electronic planners and others like the paper planners best. Find out what works for you. If you miss an appointment or have a day with too may projects, adjust your schedule.
How often do you fill out these time sheets? As often as necessary. Before signing up for new classes is a good time. You might want to redo your time management sheet right after school begins. How is it working? Do you need extra time for an algebra class. Often history and English classes are heavy on reading. Did you plan enough time for all that reading. I often renew my time sheet at the beginning of each new season. I check my daily planner every week to make sure I don't miss any appointments.
Recommended reading: "Time Management From the Inside Out," by Julie Morganstern.