Why didn't I go lucid?

Last night I had a dream that I was in my psychology class at school, (with the real teacher, room, students and everything.) and the teacher was discussing lucid dreaming. She said lucid dreaming several times and in my dream I said "Hey! I know what that is! I think I'm having one right now!" And I counted my fingers, all 12 of them :D but I didn't become lucid. The teachers discussion went on and I eventually just woke up. What happened? I've gone lucid before from a reality check, but this time it just didn't work :/

Perhaps you were not aware enough?

Sounds like you weren't aware enough. When you do reality checks in waking life, do you actually question reality and look around or you do them on auto? The purpose of reality checks in waking life is to increase our awareness and set questioning mindset which then makes us go lucid and do reality check. Also when doing reality check be sure that you believe that you might be actually dreaming and do not think anything like "Not dreaming" after doing it. Finally, putting emotions into reality check can help too, like "Wow, am i dreaming?". :)
And even if you do, your awareness might have been just too low at that certain moment, so keep practicing reality checks and awareness thus upping your general awareness! ;)

Agreed with the above explanations. Other than that, keep doing it, you will succeed. Good luck.