When someone "prepares a lesson" it means they are making the lesson. The person preparing in this case is the teacher. If you changed this phrase to someone "prepares for a lesson" that could then mean either the student is previewing the lesson (like you have guessed) or it could mean the same thing as the previous phrase (that the teacher is preparing the lesson). The second phrase depends on context, while the phrase you have asked about (when someone "prepares a lesson") is always used to talk about the teacher preparing a lesson to teach students.

Hi Gray, how would you say the first option where a student studies ahead of time, the opposite of reviewing? Preview the lessons?

A teacher prepares for a lesson. A student usually does "his homework".

For me, the only meaning of "to prepare a lesson" is the second option -- a teacher "prepares a lesson" to teach to students. I've occasionally seen other people use this phrase in ways that seemed to imply your other meaning, though, so it's possible that there are regional differences in how the phrase is understood. I'm in the US.