On average, we've seen that students tend to earn about 1.5 level's worth of experience per month (for a class that lasts a year). Over the duration of an average course, this is about 1,000–1,500 XP per month.
Naturally, these results can vary. For example, after only two months, some students may be at Level 2 while others are already at Level 4. It can also depend on your evaluation system and when you deliver grades. If you hand out evaluations and grades irregularly, students will earn XP at an irregular rate. By the end of most courses, we've observed that the majority of students end up somewhere between levels 10 and 13.
That's why few students, or none at all, reach Level 18. The only way to finish Classcraft is through complete determination and absolute participation. Only a few students ever commit to that level of involvement and studying. So in a class of 30 players, only one or two are likely to reach Level 18.
Because it's so difficult to finish the game, we strongly recommend that you find a creative way to reward winners.