You can create Ad-hoc events for your classroom from your group’s Calendar Page. Click Create in the upper left hand corner of the Calendar and choose which calendar you want to add assignment to under Calendar

After specifying the calendar you want to add the event to you can enter a Title, write a Description, choose the date/time, assign a location, schedule a repeat, and add a meeting. Additionally, you can customize your event further and add a subject line, attachments, insert images/videos, and customize formatting by clicking the button in the bottom left corner of the popup. Once everything looks good, hit Create and your Ad-hoc event will be added to the calendar.