Keep it small
Most people install ten apps in September and use two by October. The two that matter are a calendar you look at every morning and one place to write things down. Everything else is optional.
Calendar
Google Calendar or Apple Calendar. Whichever your phone already has. The trick isn't the app, it's getting every deadline into it in week one. See the guide on turning a syllabus into a calendar.
Notes
Apple Notes, Google Keep, or Notion if you like structure. Don't build a system in September. Write things down and organise them when the pile gets annoying.
Focus
Your phone's built-in Focus mode beats any third-party app. Set one for “class” and one for “study”.
Reading and PDFs
Whatever opens PDFs on your laptop. Highlighting is enough.
Before you get here
If you're still in Grade 12 reading this, the sister site cutoffs.ca has every Ontario university's admission ranges on one page, plus the supplementary application deadlines that catch people out.
SyllaBuddy
Ours, so take it with salt: it reads your syllabi and turns them into the calendar and daily to-do above, then answers questions about your courses and keeps a running memory of what you tell it. It's free while it's new. If you'd rather do the syllabus work by hand, the guide on this site shows how.