"Im more than convinced, she didn't attend the class." What does this phrase mean here? I googled it too, but the effort went all in vain.
The phrase more than basically means very. For example, these two statements are more or less equivalent in meaning:
I will be more than willing to help you solve this problem.
I will be very willing to help you solve this problem.
So, here's how you can understand your original sentence:
I'm very convinced that she didn't attend the class.