Here's a possible solution:
Highlight all your lectures, then Get Info and change the Media Kind to Podcast. This will move your lectures to the Podcast section, so that when you shuffle your music, the lectures are left out.
I think the key thing here is that the Library is not really intended to be used for playback, that's what playlists are for; the Library is "everything". My recommendation would be to create a playlist that contains all the music you want to be available to play and use that for listening to music.