One list of everything due
Collect every assignment, quiz and test date from every class into one calendar or planner. Check syllabuses and online class pages for anything you missed.
Schedule study blocks
Put specific study times in your week, like appointments: 'Tuesday 4–5 pm, chemistry problems'. A named task at a set time gets done far more often than 'study chemistry'.
Start big work early
Break projects and papers into small steps with their own dates: topic, sources, outline, draft, final. The last week then holds editing, not panic.
Do this today
Put every due date for the next four weeks in one calendar, and schedule three study blocks for this week.