Earlier this week Google added a new "add reminders" option to the Google Calendar mobile apps. If you have updated the Google Calendar app on your iOS or Android device, you should now see a red "+" button. Tap that button to add a reminder to your calendar.
The same option to to add reminders hasn't been added to the desktop version of Google Calendar, yet. There are other methods of adding reminders to Google Calendar. One option is to use HashtagToDo to add reminders to your Google Calendar. With HashtagTodo enabled in your Google Account you can create a to-do list in any of your Google Calendars by simply adding #todo to the end of any calendar event title. Once you've added the #todo to your event HashtagTodo will create a daily event listing all of your to-do items for the day. When you've completed a task on your to-do list, delete it from your calendar and it's deleted from your list. If you don't complete an item on your list for the day, it is automatically rolled over to the next day.
I'm a fan of using Google Keep to set reminders for myself. Creating reminders is just one the many things that you and your students can do with Google Keep.