Unless you routinely used a highlighter while you were studying, don't even think about using one on the LSAT. This sounds really stupid, but you have to practice using it. Fumbling between the highlighter and the pencil to darken the bubbles is a nightmare. Talk about a time waster.
I thought I might use a highlighter so when I sutdied and practed and timed myself, I tried it...NOT! Stick with the pencil, unless you have some obsessive compulsion with that yellow marker being in your hand.