It doesnt seem that you can save to the sd card directly out of snotes, but I do have a work around for you that I just tried. Go ahead and create your snote and save as normal. Then go into "my files" and cut and paste the file onto the sd card. Click on the file and it will open. Go to the upper right of the screen and hit the drop down menu. Create a shortcut to that file. Now when you hit the short cut it will open off the sd card and when you edit the file it saves back to the sd card. See if that helps.
The other thing you could do is just let your files save to the onboard tablet memory and if its getting full, move older ones to the sd card. You can always access them, you just may have to go in through "my files".
Edit: I would think this would work for lecture notes as well, but I have not used that program. Many of the android apps are set to look at a specific folder and save to a specific folder. Moving them and creating short cuts can get you around the issue although it is a little work.